Improved debugging output

Use ToString of the expression as message, this allows usage of
PartialEvaluationExceptionExpression ToString output
NH-1082 Exceptions in IInterceptor.BeforeTransactionCompletion cause the transaction to be rolled back
Rename Contributor Guide.md to CONTRIBUTION.md

So GitHub can recognize it and display on "new pull" request page
Merge pull request #208 from janv8000/master

Improved debugging
build-common/common.xml: Aiming for 3.4.
ConnectionManager.cs: Reduce direct use of ISessionImplementor.
Fix for NH-2128: Avoid firing the AfterTransactionCompletion notification twice when using distributed transactions and user-supplied connections.

The fix is by stopping the ConnectionManager from sometimes calling session.AfterTransactionCompletion(wasSuccessful, null). Such notifications should come from the transaction itself (as it already does) and in some cases from inside the session itself. Longer explanation in the bug report.

This also works to simplify the complicated bidirectional relationship between SessionImpl and ConnectionManager. The session should rely on the connection manager, but not the other way around.
Fix NH-3572: Avoid calling AfterTransactionCompletion twice when a distributed transaction is rolled back.
NH3392 - Unit test and fix
Merge pull request #239 from gliljas/NH-3392

NH3392 - Unit test and fix
NH3392/Fixture.cs: Minor corrections.
NH-3072: OneToManyPersister improvements
Fix NH-3221
NH-3221 - Correct test case to reference HashSet instead of HashedSet
NH-2756 - Add test fixture
Add support for Convert.ToInt32 / int.Parse

Also support for decimal and double.

NH-2865 : Test case added
NH-3377 : Test case added
NH-2692 - The identity for values or components is the elements themselves,
so return "elements" as property name
NH-2692 - Add KnownBug attribute
NH-3524 - Add support for DateTimeOffset properties in QueryOver
NH-3041 - Add IOneToOneMapper.PropertyReference method