Add support for store transactions

If backend adapter supports transactions (e.g. Virtuoso via ODBC)
handle transactions in backend, otherwise ignore.

The addMultipleStatements method now automatically starts a transaction
if no transaction was already started.

Also the backend method returns a boolean value whether addition was
successfull and the store class passes this value to caller.

-----------------

Jenkins is currently failing again (seems to be caused by doc stuff)... Here are the test results:

```
phpunit:
     [exec] PHPUnit 3.6.11 by Sebastian Bergmann.
     [exec] 
     [exec] Configuration read from /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/phpunit.xml.dist
     [exec] 
     [exec] ........................................................S......  63 / 615 ( 10%)
     [exec] .....................I......................................... 126 / 615 ( 20%)
     [exec] ............................................................... 189 / 615 ( 30%)
     [exec] ............................................................... 252 / 615 ( 40%)
     [exec] ............................................................... 315 / 615 ( 51%)
     [exec] ............................................................... 378 / 615 ( 61%)
     [exec] ............................................................... 441 / 615 ( 71%)
     [exec] ............................................................... 504 / 615 ( 81%)
     [exec] ............................................................... 567 / 615 ( 92%)
     [exec] ...............................................
     [exec] 
     [exec] Time: 01:33, Memory: 101.50Mb
     [exec] 
     [exec] There was 1 incomplete test:
     [exec] 
     [exec] 1) Erfurt_Sparql_ParserTest::testTokenizeFilter
     [exec] Issue #35 not yet fixed https://github.com/AKSW/Erfurt/issues/35
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/Sparql/ParserTest.php:32
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] --
     [exec] 
     [exec] 
     [exec] There was 1 skipped test:
     [exec] 
     [exec] 1) Erfurt_Rdf_MemoryModelTest::testHasO
     [exec] This test depends on "Erfurt_Rdf_MemoryModelTest::testAddAttributeCorrect" to pass.
     [exec] 
     [exec] /usr/bin/phpunit:46
     [exec] OK, but incomplete or skipped tests!
     [exec] Tests: 614, Assertions: 1087, Incomplete: 1, Skipped: 1.
     [exec] 
     [exec] Generating code coverage report in Clover XML format ... done
     [exec] 
     [exec] Generating code coverage report in HTML format ... done

phpunit-integration:
     [exec] PHPUnit 3.6.11 by Sebastian Bergmann.
     [exec] 
     [exec] Configuration read from /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/phpunit.xml.dist
     [exec] 
     [exec] ........S...............S....SSSS...........S.SS..............
     [exec] 
     [exec] Time: 33 seconds, Memory: 37.75Mb
     [exec] 
     [exec] There were 9 skipped tests:
     [exec] 
     [exec] 1) Erfurt_AppIntegrationTest::testGetStoreWithCleanDatabase
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:104
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/AppIntegrationTest.php:113
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 2) Erfurt_Rdf_ModelIntegrationTest::testIsEditableWithZendDbAndAnonymousUserIssue774
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/Rdf/ModelIntegrationTest.php:143
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 3) Erfurt_Sparql_EngineDbIntegrationTest::testOdFmiLimitQueryWithZendDbIssue782WithLimit
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/Sparql/EngineDbIntegrationTest.php:13
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 4) Erfurt_Sparql_EngineDbIntegrationTest::testOdFmiLimitQueryWithZendDbIssue782WithoutLimit
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/Sparql/EngineDbIntegrationTest.php:60
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 5) Erfurt_Store_Adapter_EfZendDbIntegrationTest::testSparqlQuery
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/Store/Adapter/EfZendDbIntegrationTest.php:8
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 6) Erfurt_Store_Adapter_EfZendDbIntegrationTest::testSerialization
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/Store/Adapter/EfZendDbIntegrationTest.php:8
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 7) Erfurt_StoreIntegrationTest::testCheckSetupWithZendDb
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:104
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/StoreIntegrationTest.php:111
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 8) Erfurt_StoreIntegrationTest::testSparqlQueryWithCountQueryAndEmptyResultIssue174
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/StoreIntegrationTest.php:140
     [exec] /usr/bin/phpunit:46
     [exec] 
     [exec] 9) Erfurt_StoreIntegrationTest::testSparqlQueryWithCountAndFromIssue174
     [exec] Skipped since other backend is under test.
     [exec] 
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/unit/Erfurt/TestCase.php:150
     [exec] /var/lib/jenkins/jobs/Erfurt-Feature-Branches/workspace/tests/integration/Erfurt/StoreIntegrationTest.php:155
     [exec] /usr/bin/phpunit:46
     [exec] OK, but incomplete or skipped tests!
     [exec] Tests: 62, Assertions: 220, Skipped: 9.
     [exec] 
     [exec] Generating code coverage report in Clover XML format ... done
     [exec] 
     [exec] Generating code coverage report in HTML format ... done
```