Switch to thread_local crate in lieu of thread_local!.

This is to work around a bug where using a thread_local! was causing
a segfault on macos.

Fixes #164.

src/main.rs

@@ -16,6 +16,7 @@ extern crate num_cpus;
 extern crate regex;
 extern crate rustc_serialize;
 extern crate term;
+extern crate thread_local;
 extern crate walkdir;
 #[cfg(windows)]
 extern crate winapi;