added failing test

to help debug issue #7
added test of passing an alternative finder function

this shows how to handle the issue #7 use case with no changes to the MinimalFastaParser code. I don't think issue warrants an code change, but since I wrote this test it doesn't hurt to add it to PyCogent.
Merge pull request #8 from gregcaporaso/issue-7

added a test while investigating issue #7