Minor cleanup in `modeling` sub-package

This is just some very minor cleanup in `modeling` that is pointed out by my editor.
I replaced all `import *` because then static code analysis works in the Eclipse editor, which is nice.

@nden Not all examples in the docs currently work (e.g. [here](http://docs.astropy.org/en/latest/modeling/parameters.html#parameters-examples)). I'll work a bit on tests and docs this week ... should I start now or should I wait for other pull requests that still will go in 0.3?