Improve composer.json compatibility

Follow the hints provided by SilverStripe developers at
http://addons.silverstripe.org/submit
Initial port to Bootstrap 3 (3.1.1)
Simplified default footer
Use the default theme instead of the inverse
glyphicons are now under fonts/
Merge branch 'dev'

Porting to Bootstrap 3 completed.
Update doc

Cite the new release is based on Bootstrap 3.
Merge branch 'dev'
Cite latest silverstrap version is 2.x.x
Merge branch 'dev'