A config object is now required to initialize tracks.
Config objects are provided for H264 and AAC.
This allows to pass parameters easily and treat AAC
parameters explicitly.
Altough not explicitly allowed by RFC2326, some servers respond with
a `Content-Type` header field that contains charset information as
described in RFC2616 (sec 14.17).
The parser currently fails to read from such connections and bails
with
`unsupported Content-Type header '[application/sdp; charset=utf-8]'`
Change this by ignoring everything after a semicolon.
