v2.2.5: improve sunrise/sunset accuracy by a few seconds

Merge pull request #21 from darkskyapp/ironwallaby/fix_astro

fix sin/cos azimuth

v2.2.3: performance optimizations

v2.2.2: convinced myself to just stick with the USNO

v2.2.1: minor accuracy bump

Support solar azimuth

v2.1.1: restore lunar.rise_and_set

v2.1.0: expose sin/cos of hour angle on observer rather than hour ang…

…le (#15)

add solar.sin_elevation, fix solar.declination

v2.0.3: bugfix