Fix bug 1112: Save content of symlinks with FSMongo

- Store data for symlinks with GridFS
- Add unit tests: This includes an Example of a tar with a folder, a file, a symlink to folder and a symlink to file

Change-Id: I3ac9cb1a3bc2dc39b58e61658a96c7f97f59c2a5
Signed-off-by: David Garcia <david.garcia@canonical.com>