Check MANIFEST.in automatically

Check all the things with flake8

add URLSafeBase64Encoder, tests for all encoders

expose and test crypto_scalarmult

The two scalarmult functions always return 0 (they should really have
been declared as 'void'). So simply assert that they've returned 0
instead of trying to raise an exception in a code path that will never
be exercised or tested.

fixes #65

Rewrites the docs on SecretBox nonces. Includes more text by @warner.

Closes #60

Now it's consistent with the documentation for public key encryption.

The NaCL pages also say random nonces are fine:
http://nacl.cr.yp.to/stream.html

Add the missing README.rst to MANIFEST.in

(it seems like generated tarballs/zipballs already included README.rst,
perhaps due to some other fix landed since this branch was created, but
it's a good idea to include it in MANIFEST.in anyways)

refs #56

fix the 'wrong keysize' error

closes #59

fix argument order of low-level NaCl functions

It also has a test vector for test_secretbox() that I generated with a C
program linked against libnacl.a .

Issue #62: Set initialized flag to true after loading library.

Make flake8 clean

Call ffi.cdef() for header files in predictable order

Closes #53.

Always call ffi.cdef() with a consistent ordering of the header files.
This avoids problems where CFFI generates different checksums (used in
.so filenames) at compile time and runtime, and tries to load the wrong
.so file.

Fixes issue #53 (https://github.com/pyca/pynacl/issues/53)