improve the hygiene of compiled functions

the variables I have namespaced are:
+ __indent -> jade.indent
+ [escape, merge, attrs, rethrow] -> straight namespace
+ interp -> jade.interp
+ __val__ -> jade.interp
+ __jade -> jade.debug

as far as I am aware the only reserved words left now are "jade" and "buf"
Merge pull request #1 from jkroso/fix/hygiene

improve the hygiene of compiled functions