Updated readme formatting

* Markdown blocks use GitHub Flavored Markdown per http://github.github.com/github-flavored-markdown/
* Fixed indentation of ruby code blocks that didn't require it
Merge pull request #73 from parndt/patch-1

Updated readme formatting
Rename Messageable#(un)read to mark_as_(un)read and Receipt#(un)read to is_(un)read
Bump version, update README.
Merge pull request #76 from rafaelgg/master

Avoid conflicts with ActiveRecord's read
Remove deprecation warning on InstanceMethods
removed incorrect :string mention in down migration. remove_column only needs the column name
Merge pull request #97 from mobilutz/master

remove column type from down migration
Added support for STI
Merge pull request #99 from bennick/master

Added support for STI
Bump version
Update lib/mailboxer/models/messageable.rb

added ability to specify timestamp to message API (https://github.com/ging/mailboxer/issues/14)
Merge pull request #101 from runtimerevolution/master

adding timestamps to message api
Fixing test errors after merged pull request #99
Merge pull request #102 from fabianoalmeida/master

Fixing test errors after merged pull request
Adding test to PR #101
Merge pull request #103 from fabianoalmeida/master

Added forgetful test
Updating contributors and fixing #104 issue
Merge pull request #105 from fabianoalmeida/master

Updating contributors
Update app/models/receipt.rb

Fix for #110