Azimuthal Equidistant projection

This is not totally complete yet, but I'm looking for some feedback. Let me preface this by saying I'm neither an expert in cartography nor the proj4 library. I have implemented the CRS, some tests, and added to the documented list of projections (note: this depends on #505).

![projections-3](https://cloud.githubusercontent.com/assets/302469/4852417/d939cb96-607b-11e4-8b79-4e11442a0fe9.png)

However, I am running into a couple minor (or not?) issues:
 * The tests are from the USGS report 1395, [Map Projections - A Working Manual](http://pubs.usgs.gov/pp/1395/report.pdf), but two of them do not work out exactly correct. I tried [implementing the equations myself](https://gist.github.com/QuLogic/0eb4afbc412212b8315a) to see if it was perhaps a precision issue in the report, but I get the same answer.
 * The continents will not always draw, depending on the projection parameters. I used the North pole as the centre for the documentation (this is the same as the UN logo), but other central points do not always work. Sometimes they are distorted, and sometimes there's not visible at all.
 * None of the internal plumbing is documented, so I'm not really sure about all the CRS methods (e.g., `CRS.threshold`).