Changes

[Feature #46] EntityManager.getReference added into the API.
 (commit: 84afb0d)

[Feature #46] Added instance descriptors which will be used for tracking
instance load state in a persistence context.
 (commit: 6606c27)

Implementing InstanceDescriptor-based isLoaded in UnitOfWork.
 (commit: c0910a1)

Minor refactoring of query factory usage.
 (commit: 5dc8b1b)

Modified internal entity loading API to support InstanceDescriptors.
 (commit: 08f41b3)

Revert "Modified internal entity loading API to support
InstanceDescriptors."
This reverts commit 08f41b3d
 (commit: 9642a87)

Tracking load state of existing entities loaded in UoW. Updating it on
field fetch and set.
 (commit: 4482445)

Use the new isLoaded functionality in PUUtil. Testing it.
 (commit: 3a93c98)

[Feature #46] Implementation of get reference on the storage access
level.
 (commit: d832889)

[Fix] Fixed failing tests in jopa-impl.
 (commit: abf7a48)

[Feature #46] Implemented loadReference in OOM.
 (commit: 233f5b0)

[Feature #46] Simplified reference loading (skip cache check).
 (commit: 258f2a1)

Use instance descriptors in UoW when determining whether to load
(lazily) fetched attributes or not.
 (commit: 51c2ccf)

[Feature #46] Started working on getReference implementation in UoW.
Decided to use a different semantics - return null instead of throwing
EntityNotFound exception.
 (commit: 9f0997b)

[Refactoring] Simplifying handling of object removal in UoW and change
sets.
 (commit: 8234318)

[Feature #46] Testing basic getReference behavior. Integrated its
implementation into EntityManager.
 (commit: 4da49a9)

[Feature #46] Further testing of treatment of references in UoW.
 (commit: 0d9693f)

Added a reference to the OTM library comparison paper.
 (commit: 49b425d)

Added a reference to the OTM library comparison paper.
 (commit: 9b65524)

[Upd] Removed finalize from EntityTransactionImpl.
 (commit: c29c40a)