srtm: implement extract_point method

Extracts the elevation of a single point from the SRTM data. Includes a
test to compare the value against Google's elevation API.
strm: extract_track method

Method to extract the elevations from STRM using a Track object. Test to
compare the calculated and observed elevations by direct comparison and
calculating RMS.
Update .gitignore
rk: split command-line specifics out of authorize method

Create a subclass containing the command-line authorization specifics.