collect: symbolic link storage

* Adding a symbolic link storage for developers. To enable it, put this
  to your config: COLLECT_STORAGE = u"invenio.ext.collect.storage.link"
  then run `inveniomanage collect` to create all the links from the
  static folders to the intance folder.
  NB: only files are symlinked, not folders!

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
base: deprecation of STATIC_MAP

* Deleting STATIC_MAP and all its links. All the files should be served
  from the instance_path directory via collect.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
travis: collection of the static files

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>