Pull Request for Issue#45 and other two other bugfix

It has 3 major enhancement and bugfix for AbstractAjaxCallback.java:

1. in function clear(), reset all local variables in order to reuse same AbstractAjaxCallback instance.

2. in function httpDo(), add code for IOException processing to remove partial content file in cache.

3. in function filePut(), add code for remove cache file when user called status.invalidate() in case of  status.getFile() is not null