Hello!
This is meant to address #128. I've added two additional methods to the Sequence object:

• gc_strict: functions as gc, but filters out non-ATGC bases.
• gc_iupac: functions as gc, but additionally considers IUPAC ambiguous bases as having fractional G/C content and filters out non-IUPAC bases.

Thoughts and Considerations:

• I used a Counter because it should (and did, in my testing) perform better than 12 calls to count().
• I didn't touch the original gc method because it's still faster, and remains accurate for most situations.
• I'm wondering if there's a more efficient way to filter out bases than regular expressions, or if there's better within re than re.sub().
• I'm wondering if there's some way to compile the expression for when many sequences are being analyzed, without having to compile it for every individual call.

I'm new to this process, so please tell me if there's anything I've misunderstood about the issue or the process of contributing. I'm more than happy to modify anything that should be changed.