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.
Same here