Abstract index building and index loading.

Summary:
 - abstract compressed index loading to IndexLoader.
 - change some constructors according to RAII paradigm.
 - closes #35.

Test Plan:
make test
bin/mws-index -I data/zbl/ -o data/zbl-index
bin/mwsd-load -I data/zbl-index/

Reviewers: radu.hambasan, m.iancu

Reviewed By: m.iancu

Differential Revision: http://phab.code4fun.de/D69