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)

Fix the permissions on all of the libsodium files

Ensure that our configure script has permissions

Update libsodium to 0.4.5

Fix up the flake8 output and include it in the test matrix

Use code-block

Make docs build

Update the documentation configuration