Update presentation of mail controller to include completion block

Since the Mail Controller popover isn't a real view and can cause issues with the status bar coloring (http://stackoverflow.com/questions/18945390/mfmailcomposeviewcontroller-in-ios-7-statusbar-are-black) the solution seems to be to use a completion block on the mail presenter method.

However the current implementation uses the deprecated `presentModalViewController`, this pull request updates that to `presentViewController` and adds support for a `completion` option being passed.