Sign then Encrypt

Just as a heads up this is wacky as shit because of some weird merging and rebasing…I'm still looking at it but if you see anything odd please let me know.
False alarm. The public key used there is indeed the recipient's public key. However, I am not sure the key exposed is ephemeral.
```python
renelvon @ merchant ~/Repos/OpenBazaar (develop) % python2
Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from pyelliptic import ecc 
>>> e = ecc.ECC()
>>> pk = e.get_pubkey()
>>> k1 = e.get_ecdh_key(pk)
>>> k2 = e.get_ecdh_key(pk)
>>> k1 == k2
True
```