fix busted cond case in cljs.analyzer/source-path
CLJS-402: runtime obtainable compiler version number

Change build script to add compiler version information to compiler.clj
and core.cljs.

In CLJ *clojurescript-version* will be bound to a map. Provide a
CLJ function that will produce a string. In CLJS
*clojurescript-version* is bound to a string.
CLJS-659: tag compiled files with compiler version

The first line of CLJS compiled files is now a comment about the
compiler version that generated the JS file. Use this information to
determine whether files need to be recompiled.