Bumped version to 2.7.1 to cut stable release.
Merge pull request #10 from ozgune/master

Added new function to hash any PostgreSQL data type. (Closes #10)
Added documentation for hll_hash_any. Fixed formatting issues in documentation.
Changed hll_add_trans() functions to return hll_empty instead of NULL when aggregating over empty sets. This makes hll_add_agg() conform to PostgreSQL's count() semantics over empty sets, and closes issue #2. Also added two small regression tests.