Improve rebar.bat

Add support for invoking rebar from other than current directory.
For example: ..\rebar create-node nodeid=mysample

requires rebar.bar and rebar (escript) to be in the same directory.

rebar.bat

@@ -1,2 +1,3 @@
 @echo off
-escript.exe rebar %*
+set rebarscript=%0
+escript.exe %rebarscript:.bat=% %*