Refactoring: Consolidate classpath extraction

This refactoring is required for next commit, which changes the
consolidated logic.
Recurse on the parent classloader (fix axel22/scalameter#22)

When searching for URLClassLoader, try falling back (recursively) on the
parent classloader. This finds a classloader in the setup used by SBT
0.13.0, and it seems a sensible default heuristic in general.
Merge pull request #27 from Blaisorblade/issue/22

Fix on SBT 0.13 by recursing on the parent classloader