Use rspec-mocks for mocking
Use new rspec syntax
Upgrade rspec to 3.0.0.beta1
Add coveralls back
Test on multiple action mailer versions
Test on ruby-head
Run simplecov locally without filter

This is a great way to find dead code, even inside the specs.
Remove dead code
Reorganize spec files

- split specs into unit and integration folder
- move fixtures and stubs to support folder
Remove encoding header
Update CHANGELOG

[ci skip]
Merge branch 'spec-improvements'