Install Firefox Webapp from local checkout

Ask Firefox to install the currently running copy of glowing bear
instead of the copy from torhve.github.io

This also fixes a bug when glowing bear is not running from a
subdirectory (or one other than /glowing-bear) which would lead to a 404
because of torhve.github.io/original_subdir being loaded instead of
/glowing-bear.

Note that installing to a different subdirectory than /glowing-bear will
lead to the icons not being found due to the fact that all icon paths
inside manifest.webapp have to be absolute.
Merge pull request #174 from tribut/webapp

Install Firefox Webapp from local checkout