Add values_callable feature to Enum

Add values_callable feature to Enum

Added support for :class:`.Enum` to persist the values of the enumeration,
rather than the keys, when using a Python pep-435 style enumerated object.
The user supplies a callable function that will return the string values to
be persisted.  This allows enumerations against non-string values to be
value-persistable as well.  Pull request courtesy Jon Snyder.

Pull-request: https://github.com/zzzeek/sqlalchemy/pull/410
Fixes: #3906
Change-Id: Id385465d215d1e5baaad68368b168afdd846b82c