Add a legend block

It'd be nice to have a legend block that showed what content each icon represented.

The attached patch adds a block to gmap.module that lists all content types with their icons, as chosen in the gmap_location settings. Users must limit the block to pages they want it to be displayed on through the block settings.

Further improvements:

• restrict block display automatically so it only shows on pages with maps (could do this easily for map/node but for views-created maps, I'm not sure of the best way)
• only show icons that are being displayed on the map (again, not sure the best way - is there a global variable I can get access to from within the block?)
• Improve the default CSS to make it prettier