Travis

In this PR
- Fixes for the tests at `folium/folium.py`
  - Added plugins directory to the package
  - Added missing properties to init template test
  - Check for `threshold_scale` size (must be <= 6)
  - Fix popup, mark_templ and vega tests.
- `.travis.yaml` file running python 2.7, 3.3 and 3.4 (via conda env)
- Runs the tests via nose