Several features and fixes

I'm currently using a fork of your library in a production App. Here's the changelog:

- Updated dependencies:
	- Elixir 1.0.2
	- httpotion 1.0.0
	- json: 0.3.2
	- timex: 0.13.2
- Removed struct keys from form data before making request
- Added support for `"Name" <email@domain>` format on `from` and `to` fields
- Added support for multiple recipients on `to` field
- URL encoded form data
- Removed unmatchable `ExGrid.Message.new` clauses
- Removed warnings (unused variables / functions)
- Bumped version to 0.2.3

I'm sorry for not yet adding any tests and not splitting the changes into separate pull requests. Feel free to merge them or suggest improvements.