Formatting.

Simply update code formatting to something that match more what
we use generally.

This does NOT alter any program logic.
Re-enable regression testing against libzmq 3.2
Check for curve capability at runtime to skip tests if needed.

Use zmq_has() to skip test that require curve capability if
this functionality wasn't built with libsodium.

This has the effect of not causing the test suite to fail in case
we didnt build libzmq with --with-libsodium.
Disable test_auth against libzmq < 4

This should allow the test suite to run correctly against
libzmq 3.2.
Remove method comments from .cpp.

This is a cosmetic patch only. Remove function/methods comments
from .cpp as they are in the .hpp (where they belong).
Fix use of uninitialized value.

Fix the uninitialized of `verbose` (defaults to false now).
Remove the ZAP_ENDPOINT define and use a private static const
char * instead. I think this is better since this is C++.
Also fix a few typos.
disable zap/auth stuff for libzmq < 4
Fix zap_requests's accessors.

Return `const std::string &` instead of `std::string` to avoid
useless copies.
Also removed inline, since compiler is smarter than us :)
Update comments so that work with doxygen.
Doxygen config update (and file renaming).

Renamed the doxygen configuration so we can generate the doc
using `doxygen` instead of `doxygen zmqpp.doxygen.conf`.

Edit configuration to properly extract all documentation.
Merge pull request #90 from xaqq/regression-test

Reenable tests against libzmq3.2 + various cleanup.