Key exchange protocol

This module implements DH key exchange using GMP and a small RSA implementation.

Notes:

Calling processes should implement memory locking, perhaps using:
  mlockall(MCL_CURRENT|MCL_FUTURE);
Not doing so could result in keys leaking to swap.
However, Android kernels lack support for swap, so this may not be an issue on that platform.

In the future, this library may implement custom allocation for GMP:
  http://www.gnu.org/software/gmp/manual/html_mono/gmp.html#Custom%20Allocation

TODO:
-pass a function pointer for fill_random to emsa_pss_encode
-hopefully, run a buildbot to test new revisions
-add a fuzzer that throws random data at emsa_pss_* code
 (to find any buffer overflows or other errors)