Improved Cljs compilation times by 4-5x

Based on http://swannodette.github.io/2014/12/22/waitin/

- added "target/classes" to :source-paths
- added :clean-targets to remove only project specific out
- updated cljs to 0.0-2511
- added :cache-analysis true to compiler settings

On my MBP (2013 2.3ghz quadcore i7), a cold compilation takes ~8.56s. Subsequent compilations ~2.9s.

With these settingsm, subsequent `lein clean` and `lein cljsbuild once` compilations take
~0.69 to ~0.72s.
added README, with cljs optimized build instructions
added README to leiningen template