Implement NumPy support (take 2)

[astrojuanlu]

This PR supersedes #24.

I just took @TheBB commit and improved the .travis.yml script to provide code coverage using Codecov and two paths, with and without NumPy. For instance, you can see here that only the NumPy path is being tested:

https://codecov.io/gh/Juanlu001/utm/src/numpy/utm/conversion.py#L5

The other things is that now the tests require NumPy, so I was not sure on how to proceed here. I will wait for your comments :)

[TheBB]

Thanks for this! I have updated my previous PR with test that should work if numpy is not available, and also rebased on master. If you rebase your commits on top of that it should be good (and you'll avoid that merge commit, too).

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@4c7c13f). Click here to learn what that means. The diff coverage is 95.91%.

@@           Coverage Diff            @@
##             master     #30   +/-   ##
========================================
  Coverage          ?   92.2%           
========================================
  Files             ?       3           
  Lines             ?     154           
  Branches          ?       0           
========================================
  Hits              ?     142           
  Misses            ?      12           
  Partials          ?       0

Impacted Files	Coverage Δ
utm/conversion.py	92% <95.91%> (ø)

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[astrojuanlu]

Thanks @TheBB! The history is much cleaner now. However, there are still some failures (I recall seeing force_zone_letter somewhere, but it got lost...!) so I gave you push access to my fork in case you want to fix them.

[astrojuanlu]

Closing this in favor of #24 again.