Restructure udev APIs

[swsnr]

Rename udev to pyudev and turn it into a package:

• pyudev.__init__ contains __version__ and apipkg definitions for classes and submodules
• pyudev._libudev contains ctypes wrapper code from _udev
• pyudev._core contains Context and Device classes
• pyudev._enumerate contains Enumerator class
• pyudev._monitor contains Monitor class
• pyudev._pyqt4 contains QUDevMonitorObserver class

All these classes and submodules are grouped in the following apipkg structure:

'Context': '._core.Context',
'Device': '._core.Device',
'Monitor': '._monitor.Monitor',
'Enumerator': '._enumerate.Enumerator',
'pyqt4': {
    'QUDevMonitorObserver': '._pyqt4.QUDevMonitorObserver'
},

Solving this issue is precondition for Python 3 support, because it de-couples to top-level package from implementation without loosing the comfortable namespace. This avoids imports of the API during installation phase, before 2to3 is run.

[swsnr]

Mostly complete test coverage required to assure correctness of refactoring. #4 is therefore mandatory prerequisite.

[swsnr]

Restructured API, closed by b2b46a6

udev was turned into a packge named pyudev. The namespace of this package
is controlled by apipkg, thus the public interface of pyudev is identical to
udev, but interally code is split into different modules. The required
apipkg module was added to the source tree and listed as install
requirement. qudev was moved into the pyudev package as pyudev.pyqt4.
Tests and documentation were updated accordingly.