Adds extras_require for zope.testbrowser, loosens lxml requirement so
projects using splinter don't need to use the latest version of it,
specially if they don't use zope.testbrowser. With that in mind, this
moves the lxml requirement to the zope.testbrowser extras.
update docs with instructions to install zope.testbrowser dependencies
Merge pull request #215 from cobrateam/lxml_requirement

Adds extras_require for zope.testbrowser, loosens lxml requirement