Changed Destructor due to ram dump issues. Made it now stop and unload a...

...ll sources

before deleteing the sources and buffers.

Changed README, small typo.

Issue #225
LowLatencyAudio.getDuration() can crash webworks
-Now added a check to early exit if the file is not present.

Issue #224
LowLatencyAudio.preload() should not succeed for unsupported formats
-Now added a check to see if said file to be preload is present in the
given path, if not early exit.