SdpSerializer.js added

SdpSerializer is a library aimed to provide possible customizable
features of session description (sdp).

E.g. To set maximum audio/video/data bandwidth:

sdp = SdpSerializer.SerializeAS(sdp, {
--- audio: 100,
--- video: 300,
--- data: 10000000
});

To set "framerate" for audio streams:

sdp = SdpSerializer.SerializePTime(sdp, {
--- minptime: 2,
--- maxptime: 20
});