Incorporate name of parent func in name of "parallel for" task function.

This gives us easier to decypher function names, a la
"par for FooBar.s0.y.yo", rather than just "par for s0.y.yo",
which made it hard to analyze profiling data from programs
with multiple Halide functions.
Merge pull request #227 from mmp/master

Incorporate name of parent func in name of "parallel for" task function.