Changing SceneInterface methods readTags and hasTags with tag filter parameter.

This pull requests addresses Issue #158. 

PS: We need to make sure this merge and install happens in sync with Gaffer,Jabuka,ieRendering,ieRig as it breaks binary compatibility.

- Added an enum in SceneInterface with bit mask values: LocalTag, DescendentTag and AncestorTag and EveryTag (for convenience).
- Changed the default behaviour of the readTags and hasTags, which would return all tags and now it defaults to LocalTag to be more consistent with the other read functions and because the other tags may not be supported by some implementations of SceneInterface (like MayaScene or HoudiniScene).
- Revisited LinkedScene behaviour and now, tags that are set in the main scene are available as ancestor tags if reading beyound link locations.
- Reviewed and added tests.