Add an encoder method for encoding the SPS/PPS without encoding a frame

This is useful if using a muxer that requires the SPS/PPS to be
available before the first frame is encoded.
Merge pull request #134 from mstorsjo/encode-sps-pps

Add an encoder method for encoding the SPS/PPS without encoding a frame
Merge branch 'master' of https://github.com/cisco/openh264 into buffersize_fix