moved CSS into a separate file

CSS is moved from being inline in Tileach.hta,
to it's own CSS file.
increased the default number of explorers from 5 to 6
moved explorers into a wrapping explorer group

Explorer panes are no longer added straight to the document,
and instead they are wrapped in an explorer group.
inverted the colours for backgrounds, so explorers are now darkest
moved tileack javascript out to it's own file, and added user scripts

Users scripts can now be used by adding them to
~/tileack.js.