Fixing Issue 3576: symbols('aa:g') should work

This pull request implements new behavior of symbols function, e.g.:

    In [1]: symbols('aa:d')
    Out[1]: (aa, ab, ac, ad)

A simple test is also provided.


The original issue: http://code.google.com/p/sympy/issues/detail?id=3576 also suggests some other features that might be worth implementing. I think that deserves new pull requests.