StaticAssets Supports Filtering and Uses Generalized FS Walker

See commit history for features added/bugs fixed.

This is mainly changes required to accomplish the processing pipeline for an application I'm building.  With this I can:

* Use StaticAssets to recursively walk through a directory
* Define a filter option which will limit what files get processed (which means I can separate the html files and not hash them)
* The directory walking functionality is generalized

There are a lot of changes here, so let me know how I should structure this and what fits/doesn't fit with your standards for the library.