Improve building and fix NIF path

[bismark]

• Use elixir_make for a more robust build process
• Update Makefile to not recompile everytime
• Fix NIF path issue

Re the NIF path, I was getting the following error when trying to use the library as a dependency:

  [warn] The on_load function for module Elixir.Excal.Interface.Recurrence.Iterator returned:
{:error, {:load_failed, 'Failed to load NIF library: \'dlopen(./priv/recurrence/iterator.so, 2): image not found\''}}

[codecov]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #23   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines          26     26           
=====================================
  Hits           26     26

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[leeicmobile]

I had the same issue with the nif path, wads going to put in a PR with just that change in will hold off if this is getting merged.

[doughsay]

Sorry for the delay! I'll take a look at this ASAP and get it merged

[doughsay]

Thanks @bismark for the updates and fixes! Version 0.3.0 has been published to hex.