Add checkversions to development buildout

Checkversions allows to quickly find updated versions of dependencies.

Example usage:

bin/checkversions -v -l 0 versions.cfg
Update Solr (3.6.1 -> 3.6.2)
Update libevent (1.4.8 -> 1.4.14b)
Merge branch 'develop' of ssh://github.com/liqd/adhocracy.buildout into develop
Update lxml (2.3.6 -> 3.0.1), libxslt and libxml2
Revert libxml2 to 2.8.0 and libxslt to 1.1.26

As the newer versions result in problems with diazo (non-terminating
requests / white pages), libxml2 and libxslt are reverted to the last
working versions.

Note: You have to remove eggs/lxml* and parts/lxml in order to be
effective (same goes for the previous commit).
Merge branch 'develop' into geo

Conflicts:
	buildout_common.cfg