added xhr.append method to add files to the existing request
added X-Last-Known-Byte header support for smart chunked upload recovery
added pause callback
Merge pull request #74 from wingedfox/master

New features (proxyXHR.append, X-Last-Known-Byte, pause event)
added current flag to abort method to abort only currently uploading file
fixed progressbar jumps on retries and added check for explicit and implicit connection aborts
Merge branch 'master' into dev
fixed formatting
Merge pull request #76 from wingedfox/master

Fixes and features
 - FileAPI.addMime; + FileAPI.accept;