Use relative pointers with (potentially) 64b offsets

Summary:
We can now create indexes of arbitrary size.

closes #70

Test Plan:
make test;
./bin//mws-index -I .data/zbl
./bin/mwsd -D Index

Reviewers: cprodescu

Reviewed By: cprodescu

CC: cprodescu

Differential Revision: http://phab.code4fun.de/D107