Added ability to generate dropdown from Enum field

Alternatively, if statement could be: if(substr($column->dbType, 0, 4) === 'enum'), however expected implemented option to be more robust across other database formats.

Created after followup on this issue:  https://github.com/yiisoft/yii/issues/292#issuecomment-44780779
Formatting changes to new enum field
Merge pull request #3676 from enigmatix/master

Added ability to generate dropdown from Enum field