Changes tox.ini in favor of tox and envs

Travis will now use tox to run tests.
This will setup travis envs to run tox on python 2.7, run pep8 tests and
run tests with coverage report.
Coveralls installed and called only in cover env.
Removes python version spec from .travis.yml - tox uses its own.

.travis.yml

@@ -1,10 +1,12 @@
 language: python
-python:
-  - 2.7
-install:
-  - pip install -q -r requirements.txt --use-mirrors
-  - pip install -q -r test-requirements.txt --use-mirrors
+env:
+  - TOX_ENV=py27
+  - TOX_ENV=pep8
+  - TOX_ENV=cover
+before_install:
+  - pip install tox --use-mirrors
+  - if [ "x$TOX_ENV" = 'xcover' ]; then pip install coveralls --use-mirrors; fi
 script:
-  - python setup.py testr --coverage
+  - tox -e $TOX_ENV
 after_success:
-  - coveralls
+  - if [ "x$TOX_ENV" = 'xcover' ]; then coveralls; fi