Add is_element_not_visible methods
Add tests for is_element_visible
Updated install doc to remove Python 2.5 as target

According to https://github.com/cobrateam/splinter/commit/75c0ec1b2a115eb361ccd5228ba0ddfc2f9e0984 support for Python 2.5 has been dropped. Here I've also generalized Python 3.0 to 3.x ... which might now be supported?
Support for Python 2.7+ only
Merge pull request #329 from mdxs/patch-1

Updated install doc to remove Python 2.5 as target
fixing Selenium's deprecated 'switch_to_frame'
Merge pull request #331 from nimbis/master

Selenium deprecated 'switch_to_frame'
Adjust select xpath to accomodate optgroup elements
Loosen Django requirement
Remove mouseout (use mouse_out)
Bump Travis firefox version
Merge pull request #335 from dustinfarris/django-req

Loosen Django requirement
Merge pull request #336 from dustinfarris/firefox-error

Fix Firefox errors
Add optgroup option select test
Fix firefox form test
Merge pull request #337 from dustinfarris/firefox-failure

Fix Firefox form test
Merge pull request #334 from dustinfarris/option-xpath

Adjust BaseWebDriver.select xpath to accomodate optgroup elements
travis: removed some environs.
forms: fixed optgroup tests.
tests: fixed zope tests.