Merge from central repo into bryhoyt's fork

Update 'Adding a new model backend' page.

File and ImageUploadField fixes

The issue at hand is in fact an error on import, and more specific exceptions are nicer to work with.

Fixes error where 'image.JPG' will be disallowed when `form.upload.FileUploadField.allowed_extensions` is set to `('jpg',)`, for example.

This is an issue for fields whose name might be changed at any point, like any image ending in "jpeg" (which will be renamed to "jpg". That would cause the actual file to be saved to disk with ".jpeg", but be saved elsewhere as ".jpg".

This example (mostly copied from the example in `examples/forms/simple.py`) shows it better than I can explain:

    import os
    from flask import Flask
    from flask.ext.admin import Admin, form
    from flask.ext.admin.contrib import sqla
    from flask.ext.sqlalchemy import SQLAlchemy

    app = Flask(__name__)
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///hello.db'
    db = SQLAlchemy(app)

    os.path.isdir('static') or os.mkdir('static')

    class Photo(db.Model):
        id = db.Column(db.Integer, primary_key=True)
        path = db.Column(db.Unicode(128))

    class PhotoView(sqla.ModelView):
        form_extra_fields = {'path': form.ImageUploadField('Photo', base_path='static')}

    admin = Admin(app, 'Photos are broken!')
    admin.add_view(PhotoView(Photo, db.session))

    db.create_all()
    app.run(debug=True)

... try uploading files with `.jpeg` vs `.jpg` extensions. It's pretty broken.

_get_dir_url() may be called without a path

one positional instead of keyord parameter
gettext(gettext()) => gettext()

ONETOMANY should not be InputRequired.

Apply `type_formatter` after `column_formatter` in list view.

Added missing translation for 'save and continue'

prettify file size format

Apply Jinja2 filter "filesizeformat" to filesize values

Fix two small code error in examples.

Merge from central repo into bryhoyt's fork

Save form_overrides into CustomModelConverter, to fix #421, in conjunction with https://github.com/coleifer/wtf-peewee/pull/20

Provide form_overrides as .overrides field to ModelConverter so wtfpeewe...

Provide form_overrides as .overrides field to ModelConverter so wtfpeewee can use it to generate forms.

Merge from central repo into bryhoyt's fork