Fixing README.rst to render correctly on PyPI.

Main changes:
- Using named links instead of anonymous
- Using an absolute instead of project relative link for
  CONTRIBUTING
- Changing duplicated link text so that they differ
- Lining up :target: with the line above for embedded img's

H/T to
https://github.com/coagulant/coveralls-python/blob/master/README.rst
for guidance.
Fixing final relative link and de-denting main description.
Merge pull request #434 from dhermes/fix-readme-for-pypi

Fixing README.rst to render correctly on PyPI.
Adding PyPI badge to README.
Merge pull request #441 from dhermes/add-pypi-badge

Adding PyPI badge to README.
Adding a non-protobuf allocate_ids method on Dataset.

Partially address #336.
Removing xrange and making test _Key return _Key instances.