Custom source path

Added support for custom source path under the `src` and `test` directory. It fixes inserting/updating the namespace and switching between test and implementation if the project has a different source layout (e.g. sources in `src/clojure` and tests in `test/clojure`). 

I'm a complete noob with elisp and this is only a first draft. Here are some thoughts I have:

* `clojure-source-nested-directory` - need a better name (maybe `clojure-path-inner-directory`?) :(
* Should `clojure-source-nested-directory` be defined with *defvar* instead of with *defcustom*? I don't think there's much use for it in the customization window. It's mainly for `.dir-locals.el`.
* Is there a way to reject the configured value with some predicate? (e.g. make sure it starts with a "/" or not allow ".." in it?

I will happily invest more time in it if required. I need the functionality and I'll be happy to learn elisp :)

Regards

Haim