Added a 'Fork me on GitHub' banner to the documentation.

Consistent with how most open source GH repos link from docs or showcase pages to the actual repository.
Added logos and reorganized docs homepage.
Allow EU dataset ID prefix
Added details about CLA for contributions to the docs.
Fixed broken link in docs.
Added contribution details to CONTRIB.rst.
Merge pull request #41 from crhym3/bugfix/eu_dataset_prefix

Allow EU dataset ID prefix in gcloud.datastore
Merge pull request #38 from jgeewax/docs-fork-banner

Added a 'Fork me on GitHub' banner to the documentation.
Merge pull request #39 from jgeewax/doc-logos

Added logos and reorganized docs homepage.
Merge pull request #42 from jgeewax/contributing

Add details about contributing to the library.