Fixing default ObjectPool for larger sizes.

Previously, setting IECORE_OBJECTPOOL_MEMORY = 2048 or larger would overflow the lexical cast, and you'd end up with 18446744071562067968
Added python binding for RefCounted::refCount().
Merge pull request #284 from johnhaddon/refCountBinding

Added python binding for RefCounted::refCount().
Merge pull request #283 from andrewkaufman/objectPoolMaxSize

Fixing default ObjectPool for larger sizes.