Reorganized parameter creation for OBJ_SceneCache nodes to avoid horrible munging of PRM_Templates.
Added tagFilter to OBJ_SceneCacheTransform

The filter is used to control visibility when expanding nodes. Available tags are presented in the drop down menu.
Allowing custom tag handlers to be registered with HoudiniScene.

Note that this does not match exactly with IECoreMaya. This is a vector of tag handlers, rather than registered by specific tag name. This allows one handler to provide a list of tags all at once, which seems more in line with the
intention of SceneInterface::readTags().
OBJ_SceneCacheTransform registers a custom tag reader with HoudiniScene.

This simply translates the tags already existing in the scene file, and does not currently allow for live editing/addition of tags. Also, tags are not maintained when using FlatGeometry mode. In the future, tag addition and FlatGeometry tags may be made available using custom parameters and geometry prim vars.
Tags can now be written with the SceneCache ROP
Merge pull request #14 from andrewkaufman/houdiniSceneTags

Scene Tags in Houdini