Add support for acting as a ScrambleSuit client.

This allows obfs4proxy to be used as a ScrambleSuit client that is wire
compatible with the obfs4proxy implementation, including session ticket
support, and length obfuscation.

The current implementation has the following limitations:
 * IAT obfuscation is not supported (and is disabled in all other
   ScrambleSuit implementations by default).
 * The length distribution and probabilites are different from those
   generated by obfsproxy and obfsclient due to a different DRBG.
 * Server support is missing and is unlikely to be implemented.