Fixed GILReleasePtr crash.

The crashes were observed at shutdown when running the GafferUI test cases. It seemed that some python objects were being destroyed _after_ python had shutdown, and the GILReleasePtr was therefore doing a ScopedGILRelease when python was finalized (or thought it was, despite there still being objects alive) - this lead to errors about NULL thread states. The Py_IsInitialized() call is used to detect this case, and prevents the crash.
Merge pull request #368 from johnhaddon/gilReleasePointerFix

Fixed GILReleasePtr crash.
Adding the export macros for proper exporting of symbols. IECore, IECorePython, IECoreRI, IECoreGL.
Adding the export macros for proper exporting of symbols. IECoreAlembic, IECoreArnold.
Merge branch 'boberfly-exports'