Minor changes for hhvm support.
Merge pull request #164 from F21/hhvm-support

Minor changes for hhvm support.
Use 1.4.4 for tests.
Merge pull request #165 from F21/use-arangodb-1.4.4

Use 1.4.4 for tests.
Added missing methods:
CollectionHandler -> checksum and revision
Database -> listUserDatabases
DocumentHandler -> head and added revision parameter to getById
Added missing methods:
Connection, enabled headers
DocumentHandler, added head and fixed get
EdgeHandler, chaned calls to not use inherited calls from documentHandler
GraphHandler, added getVertices and getEdges
new JobHandler,
CollectionHandler, new Methods, first, last, fullext
deleted JobHandler, created base methods that can be shared by edge and document handler. fixed documentation errors
fixed parse errors for PHP 5.3
updated ArangoDB version
more cleanup
fixed tests
updated and fixed docs
again...
Include early PHP 5.6 versions in testing
fix user functions test
adjusted tests for 2.0
added tests
adjusted travis script
slightly updated README