Make numpy visible in the docs

[eumiro]

For my project https://github.com/eumiro/pygohome I needed a lat/lon to utm converter to convert 100k+ of coordinates at once. Unfortunately using utm.from_latlon for individual coordinates in a loop took too much time. Only after I studied the code and history of the project, I discovered the NumPy support, which made it fast again and I'll add it to the project again.
That's why I added the mention and code examples of Numpy usage into the README.rst and the API. At the same time I completed the API doc, cleaned it a little bit, and fixed some typos. Hope you like it.

[codecov]

Codecov Report

Merging #50 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   95.00%   95.00%           
=======================================
  Files           3        3           
  Lines         160      160           
=======================================
  Hits          152      152           
  Misses          8        8           

Impacted Files	Coverage Δ
utm/conversion.py	94.87% <100.00%> (ø)

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[Turbo87]

sorry for the late reply. this looks awesome. thanks a lot! :)