Make ebloom / scalable bloom a compile-time option

bloom (from basho) uses a NIF implementation which
may be faster and use less memory; test before use.

The “old” bloom, a.k.a. “scalable bloom” 
is pure erlang, which also has nice properties. 

Note!  Switching to ebloom changes the file format
so the default is still to use ‘scalable bloom’.
Add travis build file
Merge branch 'master' of github.com:krestenkrab/hanoidb