Tabs

In the context of buliding the prototype for http://github.com/GEMScienceTools/hmtk this pull request investigates the possibility of creating plugins for the GUI aimed at adding new methods to hmtk. More precisely, it implements

0) A minimal plugin system for loading external python modules
1) Dynamic form creation. Each group of hmtk methods is managed by a `Tab` instance. In each tab the user, upon method selection, is presented a form tailored for the method with proper widgets for each given input type. The form structure is derived by the informations inspected at runtime on the method implementation class.
2) A Map "info" tool the user can use to inspect the catalogue layer and the raster one (if available).