Implicit dataset from environ

~~This has #436 as a diff-base, so feel free to hold off review until it is merged.~~
This adds helper methods for implicit datastore environ.

By setting two environment variables

- `GCLOUD_DATASET_ID`
- `GOOGLE_APPLICATION_CREDENTIALS`

the user can call convenience methods in `gcloud.datastore` directly
without worrying about auth or the name of the dataset. The
goal is that in places like App Engine and Compute Engine, the
dataset ID can be implied without **any** user intervention.

Partially addresses #337.

NOTE: This still needs to be documented, but it's unclear where is appropriate. We also need to have documentation for auth (outside of CONTRIBUTING.md).
Hmmm?  Everything inherits from object:

    $ python
    Python 2.7.9 (default, Dec 16 2014, 16:59:02) 
    [GCC 4.6.3] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> class Foo(dict):
    ...     pass
    ... 
    >>> Foo.__class__
    <type 'type'>
    >>> Foo.__mro__
    (<class '__main__.Foo'>, <type 'dict'>, <type 'object'>)
    >>> issubclass(Foo, object)
    True
    >>> isinstance(Foo(), object)
    True