Fix typo in docs

Add ability to annotate field with help text

As indicated in WTForms documentation, fields have a description, which is
typically used for help text [1].

Also, in Twitter Bootstrap, help text are supported by `p.help-block` element
[2].

[1] http://wtforms.simplecodes.com/docs/1.0.1/fields.html#wtforms.fields.Field.__init__
[2] http://twitter.github.com/bootstrap/base-css.html#forms

I guess `form_columns` docstring was copy-pasted from `list_columns` and the
occurrence of the identifier wasn't replaced.

Fix missing FileAdmin class initialization

In the FileAdmin code example the admin view wasn't really created, its arguments were passed directly to the `admin.add_view` function.

Support for nested (joined via relation) table columns, e.g. user.name

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)