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

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)
Merge pull request #54 from wbolster/issue-53

Call ffi.cdef() for header files in predictable order

Closes #53.
Set initialized flag to true after loading library.
Make flake8 clean
Merge pull request #64 from alex/flake8

Make flake8 clean
Merge pull request #63 from cmusser/master

Issue #62: Set initialized flag to true after loading library.
add tests for low-level 'nacl.c' API

It also has a test vector for test_secretbox() that I generated with a C
program linked against libnacl.a .
fix raw crypto_secretbox API
fix raw crypto_box API
fix raw crypto_sign API
test_raw.py: hush flake8
fix py3 bytes-vs-str problems
README: add Changes section to document the low-level API changes
bump version to 0.3.0 so we don't forget
Merge pull request #66 from warner/fixraw

fix argument order of low-level NaCl functions
fix the 'wrong keysize' error
Merge pull request #59 from teepark/master

fix the 'wrong keysize' error

closes #59
include README.rst (required by setup.py) in manifest
Merge pull request #56 from justquick/master

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
Remove the FUD about random nonces

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