Add one-time authentication functions

This exposes NaCL's one-time authentication functions under `nacl.onetime`. Since by definition this is *one-time* authentication, I avoided classes (and hence state) in the top-level API for now, but am very much open for comments or suggestions if there is any upstream interest.

```python
In [1]: import nacl.onetime, nacl.utils

In [2]: key = nacl.utils.random(32)

In [3]: msg = b"Foo"

In [4]: a = nacl.onetime.generate(msg, key)

In [5]: len(a)
Out[5]: 16

In [6]: nacl.onetime.verify(a, msg, key)
Out[6]: True

In [7]: msg = "Bar"

In [8]: nacl.onetime.verify(a, msg, key)
---------------------------------------------------------------------------
BadSignatureError                         Traceback (most recent call last)
<ipython-input-8-d2ff9f304af4> in <module>()
----> 1 nacl.onetime.verify(a, msg, key)

/home/pieter/envs/qauth/local/lib/python2.7/site-packages/PyNaCl-0.3.0_onetimeauth-py2.7-linux-x86_64.egg/nacl/onetime.pyc in verify(authenticator, message, key, encoder)
     65                          KEY_SIZE)
     66 
---> 67     return nacl.c.crypto_onetimeauth_verify(authenticator, message, key)

/home/pieter/envs/qauth/local/lib/python2.7/site-packages/PyNaCl-0.3.0_onetimeauth-py2.7-linux-x86_64.egg/nacl/c/crypto_onetimeauth.pyc in crypto_onetimeauth_verify(authenticator, message, k)
     51     if lib.crypto_onetimeauth_verify(authenticator,
     52                                      message, len(message), k) != 0:
---> 53         raise BadSignatureError("Authenticator was forged or corrupt")
     54 
     55     return True

BadSignatureError: Authenticator was forged or corrupt
```