New methods, events and offset option

Hi Max,

I added the following:
Events:
`error` - triggered when FileReader encounters an error reading data from the file, emits error
`end` - triggered when all chunks are successfully read and written, emits file.size
`progress` - triggered upon each write, emit current offset

Methods:
`pause` - pauses read / write, returns current offset
`resume` - resumes read / write
`abort` - aborts FileReader and ends/closes destination stream

Options:
`offset` - optional offset option, very handy in case of `abort` 

Hope this looks ok to you