Introduce Tox [#70, #69]

Make testing easier on both remote Jenkins/Travis and local, development
environments by using Tox project.

 * add tox.ini
 * use tox for distutils via `python setup.py test`
 * add requirements-dev for development requirements

1. Update readme with dev information

Add information on have to start development/hacking to readme.

2. Improve setup-env script

 * use curl instead of wget
 * skip unnecessary exports
 * skip unnecessary variables
 * handle errors
 * download to /tmp/hadoop-{distribution}

3. Update gitignore - tox/swp

Ignore .tox and *.swp files

4. Make Travis use Tox

 * matrix for each environment from Tox
 * remove unnecessary commands
 * install requirements from requirements-dev file

5. Add run_tests script

Add generic run tests script, use in both tox/setup.py and still make it
useful for manual testing. Manual tests - export HADOOP_HOME and run
script with your local HADOOP_HOME.