Work on making MouseTrap available over PyPI

In order to make MouseTrap available over PyPI and installable
using `pip` and `easy_install`, a `main` function was needed so it
could be used as a wrapper and the input function for the script.

The MouseTrap version has also been added to `__init__.py` and is
available as `__version__` so other scripts can reference it.

The requirements are dynamically generated based on whether
Python 2 or Python 3 is being used.  Not all of the requirements
can be installed using `pip`, such as PyGObject (except under
Windows), but the requirement checks can still be done
automatically.

`MANIFEST.in` is used by `setuptools` to tell it what files must
be included when packaging the application.

This will automatically set up a `mousetrap` command that can be
run on the command line through the `entry_points` section of
`setup`.
Added classifiers
Customize the `install` and `egg_info` commands

During `egg_info`, specific dependencies which must be installed
to the system will be checked.  If they are not found, an error
will be written which explains the problem and how it can be fixed
on yum- and apt-based systems.

During `install`, the translations will be compiled so MouseTrap
will be able to detect and use them.
Automatically compile locales on install
Don't display xlib warning for Py3
Do not exclude *.mo files

Previously *.mo files were excluded from the distribution as they
are generated during the install process.  Because the manifest is
checked after we generate the locale files, the files were still
not being installed and would not be picked up by MouseTrap.  With
the changes described below, the files are being generated by the
installer and still are not being included in the distributions.

During the `egg_info` command, which is run before building and
installing packages, the previously generated files are being
automatically removed.  A check has been added to ensure that this
is not done during the installation process, which would have
previously removed the newly generated locale files.

`self.distribution.script_args` is a list containing the arguments
included when the command was run.  For the installer, the first
argument is "install", which is what the script checks for.
Merge pull request #28 from GNOME-MouseTrap/setuptools

Add pip support