Added more grammar support for import statements

Hey,

I have been playing around with the grammar for cl-python trying to get to more compatable with the python 2.7.5 grammar.  This is an extension of the existing import grammar that will allow trailing commas.

``` python
from sys import (path, argv,)
```

I have been having some problems getting the test cases to re-compile as expected.  I seem to have to quickload the whole clpython.test to actually get correct results after i have had one failed run.  Otherewise it seems to report the same failure over and over.

Thanks,
Russell