working on getting travis script working
testing: use debian packages and add badge
correctly find the correct extension on python 2

before, gsw module didn't work
with new gsw machinery, fail if gsw doesn't load
port to new gsw module
plotting improvements

- allow overloading plot defaults in `add_freezing_line`
- if `contourint is None` for `plot_ts`, don't plot contours
improve plot_section command
Merge branch 'master' into travis

Conflicts:
	.travis.yml