Added possibility to specify a callback after read/write ops

With fastdom.read(fn).then(fn) or fastdom.write(fn).then(fn) one can
have a callback for read/write operations. This is (e.g.) nice to have,
if you wanna write but in first place have to read some values out of
the DOM. Therefor i had to alter the return value of read/write, which
is now job instead of id. But one can easyly get the id via job.id (var
job = fastdom.write(fn); console.log(job.id)).