Integrate mwsd-load and mws-index into mwsd.

Summary:
 - change mwsd to always build or load a compressed index
 - If -D flag is not present, it will try to build the index in memory from the harvests provided with the -I flag.
 - If -D flag is present and there is an index at that location, it will load it, ignoring the -I flag. Otherwise, it will attempt to build it using the -I flag, save it to disk and then try to load it.
 - closes #40

Test Plan:
mwsd -I ../data/zbl
mwsd -I ../data/zbl -D Index/
mwsd -D Index -I foobar/

Reviewers: cprodescu

Reviewed By: cprodescu

Differential Revision: http://phab.code4fun.de/D71