Generating a proper breadcrumbs in FileAdmin

Avoid 404 error with FileAdmin

e.g:
>>> op.normpath(directory).startswith(base_path)
False
>>> op.normpath(directory)
u'/home/projects/mywebsite/static/utils'
>>> base_path
u'/home/projects/mywebsite/admin/../static'
>>> base_path = op.normpath(base_path)
>>> op.normpath(directory).startswith(base_path)
True

Some doc improvements (with patch to fix issue #25)

Fixed sorting, searching, and make M2M optional

import from gettext from flask.ext.admin.babel

in examples: import flask-sqlalchemy from flask.ext hook

in examples: import flask-sqlalchemy from flask.ext hook to support flask-SQLAlchemy>=0.16 (this change is also backwards compatible for <= 0.16)

PK for models is optional. Some model backends might not have PK and still will work with individuals records.