Customizable jump between test-/impl ns

Hello, 

This pull request is to address the issue of non-configurable `src` and `test` directory paths in `clojure-mode` and `clojure-test-mode`, that is, the path components used by the functions `clojure-test-for-fn` and `clojure-test-implementation-for`. These are used to resolve a source to its corresponding test ns and vice-versa.

Leiningen does support customization of clojure `src` and `test` directories in `project.clj` and thus should `clojure-mode`. 

Because I've been working on a project that was organized 'the maven way' I was lacking the functionality of configurable src and test directories in `clojure-mode` above.

Thought: Alternatively source and test directory location could be extracted from project.clj. I strongly advise against it though, because clojure-mode would then depend on Leiningen enabled projects and would not work for, e.g. mavenized projects.

Cheers,
Fabio