Updated benchmark code to run potential and gz

Runs on smaller model and grid to make it more practical.
Fix test for gets_tess to check with almost_equals

Was comparing read numbers with == and assert. This could give problems
when floating point numbers don't exactly match. Switched to
assert_almost_equals for a better and safer comparison.
Merge pull request #16 from leouieda/test-gets-tess-error

Fix test for gets_tess to check with almost_equals
Changed the badges on the readme

Using the nice badges by http://shields.io/
Github release badge is not responding so removed
Merge pull request #17 from leouieda/new-badges

New badges in README
Updated changelog. Made list of known issues
Ignore vim swap files
Merge branch 'master' into bug-prism-arctan2