SERVER-2459 add --excludeCollection to mongodump

I wanted to resolve SERVER-2459, so I patched /src/mongo/tools/dump.cpp to add a new command line option, --excludeCollection.  I also updated the man file debian/mongodump.1 to reflect this.  I think I have made these changes following the style of the existing code.  This is my first contribution to Mongo and if you see issues with my code, I would appreciate any feedback.  Thanks for considering my patch.