Add ActionMailer file delivery method

OK let me try that diff again this time with the commits the right way around :)

I like this feature.

The rdoc for :location mentions sendmail... Could you revise that bit?

OK, here's another patch.

when i try to apply the patch, i get
Patch does not have a valid e-mail address.

+1 This patch will make testing email rendering a lot easier in development and could replace the role of the inaction_mailer gem.

I've added some additional enhancements to the patch:

• Try to use the tmp directory in Rails Root and fallback to the system tmp directory
• Create the directory for the mail if it doesn't exist
• Write emails to users in the To, CC, and BCC field instead of just To

I've attached a test.

I've attached a patch for the master branch also.