implement a hashring library

This implements a basic "hashring" thing, which I intend to use to build a new hashing option (retaining ketama obviously). The hash function is based on murmur, which I chose because it's faster than md5 (what ketama uses) and can be written portably in C code. There are some faster hash functions but I didn't want something weird that relied on crazy vectorized code or inline assembler.

Note that while this code is not being used yet, it's not being linked into the statsrelay binary, so it should be fine to merge.

I've tested the `test_hashring` binary with valgrind to ensure that it's correct wrt memory leaks.

I want to get feedback on this before submitting more code that uses this.