fix some typo and pep8 breaking rules
dynamic version of the library on __init__
fix import recursion introduced with the user agent support functionality
updates way to represent headers
the previous change takes place in this commit
typo in docstring
add support for default headers sent for future http_signature middleware
add direct import of middlewares at root of the package
entry points from 'britney.plugins.middleware' group are now loaded on top of britney.middleware package