Apifriendly - enforce api rate limitations

[davehayes]

I needed to write this for my own code, as I want to be friendly to the MusicBrainz servers. Thus I will be maintaining these for quite a while as it is how my code accesses MusicBrainz now and for the foreseeable future. In the spirit of open source, I offer these changes to the original repo if they are wanted.

This pull request adds the following features

• automatic api rate limiting in fractions of a second
• an optional cache, assuming one-to-one correspondence between a URL and results in a small window of time. This is so you don't make the same lookup more than once.
• a user agent string or you can just provide an email
• a non-blocking promise interface to search (and with rate limiting, this technique is ok to use now)

I'm open to comments or changes, or you can just completely ignore this pull request as ravings from a mad developer. ;)