Added BW::Mail module (@Swatto's motion-inappmail with tests)

This is the motion-inappmail gem from @Swatto, modified to work with BW and some of the syntax simplified.

https://github.com/Swatto/motion-inappmail/issues/1

I need it in my app and would like it to be in BubbleWrap as @clayallsopp suggested.

```ruby
# Opens as a modal in the current UIViewController
BW::Mail.compose self, {
  to: [ "tom@example.com" ],
  cc: [ "itchy@example.com", "scratchy@example.com" ],
  bcc: [ "jerry@example.com" ],
  html: false,
  subject: "My Subject",
  message: "This is my message. It isn't very long.",
  animated: false
} do |result, error|
  result.sent?      # => boolean
  result.canceled?  # => boolean
  result.saved?     # => boolean
  result.failed?    # => boolean
  error             # => NSError
end
```

Thanks!
Jamon