Added support for Rails 4.1+ enums.
Merge branch 'feature/enum' into feature/enum-master

Conflicts:
        README.md
:enum configuration now can be omitted if AASM column points to an
integer column. :enum can be set to true; in this case AASM would infer
enum method name from AASM column name.
Minor fixes / additions to the README file (enum-related section)
README formatting fix.
Merge remote-tracking branch 'origin/pr/124'