Added two new packages and modified the Bootstrap package a bit

@ekazakova, pls review. :-)

 - Added .gitignore file
 - Modified Bootstrap package structure; added SASS; modified Bootstrap Sitefinity styles for the navigation widget;
 - Added Foundation framework package
 - Added SemanticUI framework package
 - Added share directory for assets that are shared between different packages.
 - Updated documentations for the three packages.