Added $ie-property-hack-prefix option

Added setting to allow the prefix used for IE6/7 propery hacks to be
customised.
Documentation for $ie-property-hack-prefix
Removed $ie-property-hack-fix

Reverted to a hard-coded IE6/7 hack prefix, but now using a start-hack
instead of bang-hack.
Merge pull request #162 from djessup/master

Fix for issue #161