Email Attachment support for Android

Hi Stuart,

I have implemented the code to send an attachment in Android.

Because I need access to the Activity (to store files in a way the email program can access the file, that was the hardest thing to do), I have made DoOnActivity protected (was: private) in MvxAndroidTask. An alternative is to do the resolve to IMvxAndroidCurrentTopActivity myself. 

Sadly enough there seems to be no solution where you can keep the attachment in memory only, you have to create a file on disk.

With this code you can send only one attachment (that fits best in the most standard Android email implementation for email apps).

Best regards,

Hugo