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)
For all of these cases, we want to use [`timezone.now()`](https://docs.djangoproject.com/en/1.7/ref/utils/#django.utils.timezone.now)

```
from django.utils import timezone

foo = timezone.now()
```