Update Fetch class to use option Hash

Convert the 2nd and 3rd args of Fetch.new() to a Hash. The new options are:

:byte_start (Fixnum >= 0)
:byte_end (nil, Fixnum > :byte_start

This allows you to specify them as needed, and not need to provide other useless args
Merge pull request #23 from theckman/fetch_opts

Update Fetch class to use option Hash