Added tox.ini to project root. NOTE.

NOTE: The Python package "tox" automates the process of installing and testing
any setup.py-enabled Python package under a variety of environments at the
same. This means we can automatically test the process of creating fresh
installs of Abjad (including dependencies) under both 2.7.x and 3.4.x and then
running py.test against them.

Here are the current contents of tox.ini:

    [tox]
    envlist =
        py27
        py34

    [testenv]
    commands =
        py.test
    deps =
        configobj
        ply
        pytest
        sphinx

    [testenv:py27]
    deps =
        configobj
        enum34
        ply
        pytest
        sphinx

To run tox on Abjad, navigate to Abjad's root directory and simply invoke tox:

    abjadroot$ tox