fixes for jslint

removed quite a lot of warnings
fixed #51

CustomEvent polyfill for browsers that don't support Event.
integrated pull requests #58 and #59

- #58: Let imgcache.js work with encoded URI's
thanks ryreitsma
- #59: amd & browserify imgcache
thanks activars
added onprogress callback on download (#54)

+ updated readme and changelog with changes in this release
added getCachedFile method (#55)

+ added unit test + doc