Add OGLE-III interstellar extinction calculator query

I've made a first pass implementation of the OGLE-III extinction calculator (http://ogle.astrouw.edu.pl/). With the aim to include other extinction queries, I think this is a first step, and at a later date a wrapper can be made to combine all the extinction related queries. 

The module currently doesn't meet the full API, but it includes a docs page, doc strings, and should meet the general API guidelines. The module doesn't need to store state and only executes one query, so it should be simple to refactor at a future date. 

There is a hack-y function `ogle._parse_coords` to implement searches with 1) one `astropy.coordinates` instance, 2) a list of `astropy.coordinates` instances, or 3) a 2xN list of lons and lats as strings or floats.

Thanks in advance for any review of the code!