Add port to show index of granted signal

The sel port displays the index of the currently granted signal.
This is useful for directly driving multiplexers

Signed-off-by: Olof Kindgren <olof.kindgren@gmail.com>
White space formatting for arbiter.v
Rename arbiter 'sel' -> 'select'

No real need to shorten an already short variable name. Also add it to
the testbench waveform.
Vertical code alignment and rearrangement
Merge branch 'olofk-master'

Merge pull request from Olof Kindgren to add a binary number
representation of the 'grant' port list signal. A small amount of code
reformatting and refactoring was done before inclusion.

This new signal 'select' can be optionally used instead of the 'grant'
signal.
Update arbiter header comments to include 'select'