Add beginnings of font-face testing.

This code borrows heavily from puppet-mode-test.el, which can be found at
https://github.com/lunaryorn/puppet-mode.
Merge pull request #57 from elixir-lang/font-face-tests

Add beginnings of font-face testing.
Add CONTRIBUTING file.
Fix error in README.
Use string syntax highlighting for regex patterns.
Merge pull request #58 from elixir-lang/regex-string-format

Use string syntax highlighting for regex patterns.
Update string delimiter entries in syntax table.