Treat rtpmap rate parameter as optional

[DuBistKomisch]

Addresses #30.

[clux]

Nicely done. Parsing tests look good. Thanks for adding an extra sdp for this.
The writer string is untested, but can add it to the compose test to lock that down as well.

Regex is getting a bit unweildy, but guess that's what we get for treating two things as optional.
I'll publish a version on saturday, for now rely on the github version.

Thanks a lot.

[DuBistKomisch]

Well you know what they say about Regexes...

Thanks for merging and publishing!