Complete the README

Fix bad link (Fix #45)
Add documentation about set_dark.sh and set_light.sh (issue #46)
Add documentation about themes
set_dark and set_light apply theme for default theme

Before, set_dark.sh and set_light.sh used by default the profile name
"Default". Problem is that it is not always the default theme, so now it
will use the active theme.
Project more modular
Fix bug with gconf
Adapt the README to the last commits
Fix possible bug with dirname
Merge remote-tracking branch 'upstream/master'
Some alterations from upstream