Allow Travis to run test_plot.py

Travis is now able to execute full test suite, including test_plot.py
Change:
 * Added matplotlib and basemap as environment dependencies.
 * Install pykdtree as an optional dependency using setup.py
 * Travis installs system dependencies (libgeos) from ubuntugis ppa, latest basemap and matplotlib from git repos and executes full test suite
 * Added a link to the build status badge :)