Support CLJX in info

If file was autogenerated using CLJX, Emacs C-. should visit original file.

This patch checks local file, reads last 1024 chars and if it finds cljx stamp, it returns the name of original .cljx file. There is no dependency on CLJX.

Two questions:
1) is this right place where such functionality should be implemented?
2) is the implementation OK (performance)? For now it check files for all 'info' calls. No caches. Regardless if a project uses .cljx. 

The name of this function seems a bit confusing. Something like `handle-cljx-sources` or similar might be a better name.