Add Enum support in keyset

[vpachta]

• add expression for converting enum to it's underlying type

Closes #29

[vpachta]

Implementation for issue #29
Tested with Postgress + Sqlite DB.

[vpachta]

Thanks! Overall this looks good, but I have requested mostly a few minor changes.

Other than that, there's one concern I have regarding the equality part of the expression generation. Usually, we either build a comparison (enum conversion being applied here) or an equality (not being applied). I would've assumed some tests would fail because we're not applying the same enum conversion logic on the equality part (which should be the case in the test case you added), but it seems to be passing. So not sure if this is working properly in some of these cases.

Actually equal expression works fine for Enums it does not require to convert it to underlying type. The problem was only for comparision and that is why I did not put it for equality part.

[mrahhal]

Thanks!