Senate leaders, joined by Texas oil billionaire T. Boone Pickens, unveiled a bill yesterday to jumpstart the production of vehicles that run on natural gas. The legislation, dubbed the NAT-GAS bill, was introduced by Sen. Majority Leader Harry Reid, D-Nev., and Sens. Orrin Hatch, R-Utah, and Robert Menendez, D-N.J.It would increase tax credits for buying a natural gas-powered vehicle from $5,000