Add support for ScrambleSuit packet length obfuscation (Part of #8)

Behavior is different from phw's python code in the following ways:
 * WELL512 is used instead of MT19937
   * Instead of whatever python does to seed MT19937 based off a binary blob, obfsclient uses HKDF-SHA256 to expand the seed to the 64 bytes that WELL512 requires.
 * When sending frames, obfsclient will attempt to piggyback padding on the last frame if it will fit cleanly.  phw's implementation always sends at least 1 padding only frame and appears to never send frames with payload + padding.

Per phw, the fact that the obfsclient has different behavior than the obfsproxy scramblesuit code is not a problem.