Murmur hash implementation

This pull adds support for [MurmurHash3](https://code.google.com/p/smhasher/wiki/MurmurHash3).  It is a [widely used](http://en.wikipedia.org/wiki/MurmurHash#Implementations) non-cryptographic hash designed for hash-lookup.

The [reference test](https://code.google.com/p/smhasher/source/browse/trunk/KeysetTest.cpp#9) for MurmurHash is fairly complicated.  It first hashes integers from 0-255 and puts the results into an array.  Then hashes this entire array and uses that final hash as it's test.  Replicating this in R seems like it would be tricky, so I have not done so.

However I have generated a number of sample hashes with a [python implementation of MurmurHash3](https://github.com/hajimes/mmh3) and used those values in the tests.  All the values I have tested have matched, so I believe the R implementation is correct.

Let me know if you need any additional changes and I will be happy to do them.

Jim