Update development and CI configurations (closes #12)

* Add suport for code analysis
* Add support for Coveralls
* Test under Plone 4.1 (Python 2.6) and Plone 4.2/4.3 (Python 2.7)
* Update package classifiers
Add Coveralls badge
Do not test on Dexterity if Plone < 4.2

collective.z3cform.widgets dependency is not compatible with Plone 4.1
Pin-up version of collective.js.jqueryui to 1.8.16.9 under Plone 4.2

This is required by collective.z3cform.widgets
Skip Dexterity tests if "dexterity" extra was not included