Added .travis.yml (for travis-ci) and tox.ini files

Targets:

* Runs the unit-tests for python 2.7
* Tests unit-test coverage
* Try to build docs
* Code style checks:
  * pytest-flakes
  * flake8
  * py.test-pep8

Codes changes:
* Fixed tests for httpdownloader (using tmp dir)
* Implemented a couple of tests for Stats plugin but they fail to run on travis

Issues:
* Docs dependencies on libtorrent fails...
* Can't get py26 to work because of installing libtorrent through apt and
  the option system_site_packages fails for 2.6.