Choose stable hash function

[andychu]

Criteria for choosing, in order of importance:

1. Should be stable, and well defined
2. Should be available in all languages (e.g. JavaScript could be an issue), or easy to implement in all languages.
3. Should be reasonably fast to simulate

Candidates:

• md5 (since it's stable, and available)
• murmur hash

Ruled out:

• sha1: we are using this for the demo, but it seems to imply you need a cryptographic hash function, which we should avoid.
• city hash: not stable/versioned

This probably deserves a doc... i.e. we should go through the rationale for the choice, possibly with some performance tests.

Note: the simulation can be profiled. Random number generation seems to be a bottleneck much more than hashing.

[andychu]

Decided on md5 -- used in Python/Java/C++.