Updating Eclipse guide for new versions of m2eclipse and jclouds-labs

Changes
*  Recent versions of m2eclipse [do not support](http://maven.40175.n5.nabble.com/Releasing-maven-eclipse-plugin-td211330.html) project configurations created by the Maven Eclipse plugin. Updated docs to reflect this
*  Added instructions for optionally cloning and importing jclouds-labs
*  Misc small formatting cleanup

See also the following [jclouds-dev thread](https://groups.google.com/forum/?fromgroups=#!topic/jclouds-dev/clE7QBJl7yg)

Formatting preview (no CSS, though) in [the fork](http://demobox.github.com/jclouds.github.com/documentation/devguides/using-eclipse/). Seems to be rendering correctly.