Neo4j::Config can now be used to set id_property globally, issue #354

Cleaned up some code - create index in a callback which is called when session is available, #388
Fixed RSpec problem on JRuby #354

The RSpec did run without a database session for some RSpecs under JRuby for some reason. Fixed it by only starting session once and never shut it down.

I think the RSpecs runs much faster now on JRuby.
Rails config should set Neo4j::Config, closes #392
Added documentation and updated blog example app with examples of usage of id_property

Fixed problem with number of parameters. Some methods takes a session as last parameter. Should we remove those parameters since it might not work ?
Merge pull request #396 from andreasronge/id_property_fix

Fixed problems with id_property - user defined primary keys