Support alternate hash algorithms

This PR allows for alternate hash algorithms to be used to obscure the email address. It is backwards compatible if one is not specified, and will use hmac if a hash_secret is provided in the .ini file.

It adds gravatar support so the email address will be hashed the same way a gravatar email will be, and be compatible with [this implementation](https://github.com/kanso/gravatar/blob/master/gravatar.js#L17).

Usage
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Add the following to your .ini file

    hash_algorithm = gravatar

The hash_algorithm can be one of: none, hmac, gravatar. If hmac is selected, a  hash_secret must be provided as before.


This PR has passed the test suite :) I have yet to fully test using build_couchdb as build_couchdb is not recognizing *any* plugins.