Prevent None being converted to string and causing confirmation pop-up

Fixing the action decorator import statement

fixed problem with wtf.FileField not getting updated with request.files data upon instantiation of Form

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

fixed problem with wtf.FileField not getting updated with request.files data upon instantiation of Form

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