Squash all mailboxer migrations into a single one

Mailboxer user doesn't need to know about internal details of mailboxer
development history. This task was half implemented in commit
1901023668d087b0bd4ca498e63e5c1c0779e001, but for some reason only some
migrations were merged into the first one. Merging this would also close
PR #140.
rename module to avoid collision when subclasses inherits from AR::Base
Merge pull request #207 from apneadiving/master

rename module to avoid collision when subclasses inherits from AR::Base
Merge pull request #204 from deivid-rodriguez/squash_migrations

Squash all mailboxer migrations into a single one
fixed receipt namespace
Merge pull request #208 from overovermind/master

Added missing namespace
fix #194
code tweaks
Merge pull request #209 from apneadiving/master

fix + code tweaks