Bunch of fixes to Oracle backend

1. Implemented ```oracle_statement_backend::get_affected_rows()```.
    A new test case from pull request #59 revealed unimplemented functionality.
2. Fixed all tests for Oracle backend.
   Test for binding user defined type as input/output parameter to stored procedure call
   was broken, see commit 22e740a6bd1c120c3608dbaaf57a22b425937e65.
   Test suite was refusing to start after an unsuccessful run,
   renamed test store procedure: "soci_test" to "soci_test_proc".
3. Backend specific: added an option to fetch decimals as strings.
   When binding into row now there's an option to fetch decimals as strings
   instead of floating point (double) that may lead to loosing precision.
   This option is passed through the connection string parameter 'decimals_as_strings=1'