recommend.cc uses murmurhash

Here is the recommend.cc change without introducing recommend into RunTests.

- switched to murmurhash
- minor cleanup of --bs testcases
- rewriting recommend.cc using --topk reduction (separate pull-request) needs to wait for --rank to work as a reduction.
- even after using --topk, recommend will be generating examples and filter the previously seen ones using the bloomfilter as this seems not general enough to put inside of vw.
- Still some testcases for the library examples would be useful.