Move autodetected NAT64 prefixes above manual ones

I figure the options make slightly more sense in this order.
Add an option to make the toolbar icons grayscale

See Github issue #3: Address bar icon stands out too much. Defaults to off
because grayscale toolbar icons look really out of place to me and I don't
want to contribute to that.

This is Firefox 35+ only because that's when filter support was enabled
(see Bug 1057180.)
Simplify Makefile

Half of this stuff had never even been used for this extension.
Fix mapped IPv4 address regex

regex needed end anchor for proper detection. Was causing false positives on addresses with double colons earlier in address, eg:
2600:3c03::f03c:91ff:fe73:b7f3 was displaying as 2600:3c03::240.60.145.255
Merge pull request #10 from nivex/patch-1