Detect changes in namespaces split over several files

Addresses open issues #1, #7, #8, and #9

* will now re-load namespaces if a source file that is explicitly loaded changes.
  This will only work if the first non-comment form of the 'helper' or
   'extension' source file is an (in-ns) form. It will not look for any
   subsequent in-ns forms in the file. This is the same idiomatic usage of
   load/in-ns that is found in clojure/core.
    * newer-namespace-decls returns a list of ns-decls (as before) but also
   a set of namespace names (symbols) that have been affected.
*  avoid calling .fileModifed twice for each source file
* replace contains? (as in
   https://github.com/ahjones/ns-tracker/commit/164822959e47c2d8d2540aa063140f2a9609bfbc)
*  use tools.namespace 0.2.2 (similarly to
   https://github.com/jakemcc/ns-tracker/commit/6884b4cd8ad6e8547ce5ed6f54e3f257c547d215)
* use clojure 1.3.0 (required by tools.namespace)