Fix sphinx warnings

I had some unescaped `*`s in a couple of docstrings. Sphinx raised a warning about starting an inline strong without closing it. Adding `\`s makes Sphinx happy.
Add method to retrieve all documents

While calling `Model.find()` without any arguments will accomplish the same goal, `Model.all()` is being added as a wrapper because using `all()` can be more explicit.
Apply default sorting to QuerySets

A model's `Meta` class can take an attribute called `sort` that defines its default sort. When querying through the `find()` method, the sort will be applied to the `QuerySet`.

I feel there is probably a better way to do handle this that will prevent multiple sorts from being specified when `QuerySet.sort()` is called after `find()`.
PEP8

PEP8 was the motivation for this commit, but I'm also changing some quotation marks for consistency.
Add membership test method to Model

A `__contains__()` method is being added to `Simon.Model` to make it possible to test for a key in a document through the membership test operators (`in` and `not in`). While it is still possible to use `hasattr()`, there are times when it may more aesthetic to use `in` (e.g., when checking for both the presence of an embedded document and one of its keys).
Move geospatial helpers to their own module

The original intention of the `query` module was to house the `QuerySet` class. Over time it grew to contain `Q` and the geospatial helper methods as well. While I'm opting to leaving `Q` in the `query` module I decided it was more appropriate for the geospatial methods to be placed in their own module.

By adding a `geo` module, all methods associated with geospatial queries can be found and viewed without being mixed in with the unrelated `Q` and `QuerySet` classes.