Solution for issue #36, Valid Parenthesis in python

[bpbrum]

Dear the-vampiire,
this is my solution for Issue 36, Valid Parenthesis, in python

Approach / Though Process
From the start, I felt pairwise comparison was the way to go, and indeed it evolved naturally. I decided also to add a test against empty strings right at the beginning to avoid the burden of making all the comparisons if it was supposed to return True anyway. I re-used my "testing" function from the PR for Issue 3, but I feel it could be improved.

Problems during the solution process
As mentioned before, the solutione evolved naturally, and I had basically no problems.

Instructions to run
The file is coded to run the tests on import or open (harcoded into name == "main"), but the function has_valid_parens can be imported for testing. The input is a merely a string.

-- bpbrum