updated URDF to incorporate the indigo specific changes and fixed the base of the robot to the world
Added gravity tag to the URDF
Move base-to-world link to static tf in sim launch

Remove URDF link/joint for base-to-world link, and move it to a static transform publisher in the sim launch files
Merge pull request #57 from RethinkRobotics/indigo-devel

Tested this PR in its current state, and things work fine, so merging and creating Issue #61 to evaluate tags in the future.