Add search paths to look for sifters in multiple places
This enables the option to have sifters outside of the installed
application.  It searches the installed location, then your home
directory (~/sifters/) then the current working directory ( $(pwd)/sifters)
and if there is name collision prefers the closer sifter.

Resolves #2
Merge pull request #8 from mitocw/cg/add_sifter_search_paths

Add search paths to look for sifters in multiple places