splitit to splitit2

[mle2718]

This is basically a find-and-replace for splitit to splitit2, plus renaming of splitit.ado, splitit.sthlp, and test/splitittest.do.

I tested this by doing

net install <localfolder>
do /test/crossvalidatetest.do

which seems to work properly.

I might have fixed a small bug in the xvtests.do file.

However /test/xvlootests.do

produces this message:

. // Test that error code is thrown for missing metric
. rcof "xvloo 0.7, pstub(pred): reg wage i.industry" == 198

rcof:  _rc  == 198 *NOT TRUE* from
       xvloo 0.7, pstub(pred): reg wage i.industry
       _rc == 1000
r(9);

I think this addresses #16