Simple Enum support (#502)

[luckydonald]

This is a pull request for tracking the work for #502.

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The way it works is actually quite simple:

In python, enums are a subtype of both Enum, and a base type (int, str, bool, etc.)
In other words code:

value = SomeEnum.SOME_VALUE

isinstance(value, int) == True

This means we can simply use those values which is backing the enums as database values.

All this PR does is checking the enum type for what the underlying type would be processed as (self._get_real_converter(py_type, attr)), and store that converter internally (self.converter).

Now all the fancy converter methods (py2sql, sql2py, val2dbval, …) will be forwarded to that chosen converter.

That's it.

Bonus feature: It chooses the size of the integer or string colums correctly if you don't provide those.