Add support modified_time, created_time, and accessed_time

### What is the problem / feature ?

The `InMemoryStorage` class did not implement `url`, `modified_time`, `accessed_time`, or `created_time`

### How did it get fixed / implemented ?

- Added the ability to specify a `base_url` during instantiation if you want support for urls.
- Added timestamps for `modified/accessed/created`.
- Tests to support these new features.

### How can someone test / see it ?

- Instantiate `InMemoryStorage` and check the timestamps on the in-memory-files

*Here is a cute animal picture for your troubles...*

![pumpkindog](https://cloud.githubusercontent.com/assets/824194/4759790/50f0604c-5aea-11e4-9fb2-c2b306287923.jpg)
Late night coding.  Update when I get to my computer.
On Oct 24, 2014 7:07 AM, "Michael Warkentin" <notifications@github.com>
wrote:

> In tests.py:
>
> > +
> > +        self.storage.save('file', ContentFile('test-again'))
> > +        new_modified_time = self.storage.modified_time('file')
> > +        self.assertEqual(new_modified_time > modified_time)
> > +
> > +    def test_accessed_time(self):
> > +        self.storage.save('file', ContentFile('test'))
> > +
> > +        self.storage.open('file', 'r')
> > +        accessed_time = self.storage.accessed_time('file')
> > +
> > +        time.sleep(0.1)
> > +
> > +        self.storage.open('file', 'r')
> > +        new_accessed_time = self.storage.accessed_time('file')
> > +        self.assertEqual(new_accessed_time > accessed_time)
>
> This should be self.assertTrue.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/waveaccounting/dj-inmemorystorage/pull/4/files#r19337167>
> .
>