Release 0.8.0

* Add example of using HTML layouts with Bamboo.Phoenix ([beam-commun…

…ity#173])

* Give suggestions for why email preview may not be working ([beam-community#177])
* Add Mandrill template support ([beam-community#176])

* Build mailer config during runtime. Allows for configuratin with Conform ([beam-community#170])
* Fix "leaking" HTML email styles ([beam-community#172])
* Catch `nil` email addresses when used in 2-item tuple ([beam-community#151])
* Remove `ExMachina` from dev/prod deps. It should have been a test only dep ([beam-community#198])
* Small typo fixes ([beam-community#199])
* Explicitly set content type in email preview so that HTML emails are always preview as HTML ([beam-community#203] and [beam-community#204])

[beam-community#170]: beam-community#170
[beam-community#173]: beam-community#173
[beam-community#177]: beam-community#177
[beam-community#172]: beam-community#172
[beam-community#191]: beam-community#191
[beam-community#151]: beam-community#151
[beam-community#176]: beam-community#176
[beam-community#198]: beam-community#198
[beam-community#199]: beam-community#199
[beam-community#203]: beam-community#203
[beam-community#204]: beam-community#204