Add 1.5.1 prerelease mappings
Add 1.5.1 obf<->num<->cb mappings based on mcp744pre2

cp ~/minecraft/1.4.x/mcp744pre2-clean/conf/server.srg 1.5.1/obf2num.srg
py chain.py 1.5.1/obf2cb.srg 1.5.1/obf2num.srg | sort > 1.5.1/num2cb.srg
py reverse-srg.py  < 1.5.1/num2cb.srg | sort > 1.5.1/cb2num.srg
py reverse-srg.py  < 1.5.1/obf2num.srg | sort > 1.5.1/num2obf.srg
Add chained 1.5.1 <-> 1.5 mappings

py chain.py 1.5/num2cb.srg 1.5.1/num2cb.srg | sort > 1.5.1/cb-to-cb15.srg
py chain.py 1.5/num2obf.srg 1.5.1/num2obf.srg | sort > 1.5.1/obf-to-obf15.srg