Add DSSP unit test

Followup to https://github.com/biopython/biopython/pull/201

* I altered the original method of writing DSSP output to a tmp file and then reading it to loading it directly into memory; DSSP files are relatively short compared to PDB files (roughly 1 line per residue) so I don't think memory should be an issue. 

* The tests are very simple, just checking that the number of lines are as expected. Mostly it's just testing that there aren't exceptions thrown; I didn't want to do more specific error checking because I'm not sure how much variation there is between different dssp implementations.