Implementing parser for snapgene .dna file format

Parsing the segments described in the specification.
I've added tests; test files and the script used to generate them can be found in the testdata folder in tests.

Usage: python snapgene.py mySnapGeneFile.dna
It would be great to see ["custom" exceptions](http://en.wikibooks.org/wiki/Python_Programming/Exceptions#Custom_Exceptions) here, so calling code can reason about what happened in the parser. For example:

```python
class MalformedSnapgeneException(Exception):
    pass
```