Missing README.rst from MANIFEST.in

include README.rst (required by setup.py) in manifest. fixes the error I get when running tox

```
Unpacking ./.tox/dist/PyNaCl-0.2.3.zip
  Running setup.py (path:/var/folders/pm/r0znwdyx0274ynjsct86d16w0000gp/T/pip-ZW6sf7-build/setup.py) egg_info for package from file:///Users/jquickadmin/Projects/crypt/pynacl/.tox/dist/PyNaCl-0.2.3.zip
    Traceback (most recent call last):
      File "<string>", line 17, in <module>
      File "/var/folders/pm/r0znwdyx0274ynjsct86d16w0000gp/T/pip-ZW6sf7-build/setup.py", line 206, in <module>
        long_description=open("README.rst").read(),
    IOError: [Errno 2] No such file or directory: 'README.rst'
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):

  File "<string>", line 17, in <module>

  File "/var/folders/pm/r0znwdyx0274ynjsct86d16w0000gp/T/pip-ZW6sf7-build/setup.py", line 206, in <module>

    long_description=open("README.rst").read(),

IOError: [Errno 2] No such file or directory: 'README.rst'
```