Email: Add Attachments

[cjlotz]

Add the ability to include attachments as part of the email.

[cjlotz]

I've implemented support for adding attachments in the attachments-refactor branch. I've not merged this back into master as I'm looking for feedback. The attachments refactor introduces a few breaking changes to the existing API and I want to get some further feedback on the proposed changes.

Stuff that has changed in v2:

• Added IEmailMessage abstraction. Renamed EmailMessageRequest to EmailMessage and made internal. All "complex" email messages now need to be constructed using the EmailMessageBuilder. Alternatively use the simple SendMail API.
• Added IEmailAttachment abstraction
• Dropped Lotz.Xam.Messaging.Abstractions namespace - use Lotz.Xam.Messaging namespace
• Added platform specific WithAttachment overloads to EmailMessageBuilder to cater for platform specific addition of attachments

Let me know what you think