Remove spurious assertions about Content-Length header value.

They are 'magic numbers' which break when code-not-under-test changes.
Merge pull request #275 from tseaver/remove-content_length_assertions

Remove spurious assertions about Content-Length header value.
Basic regression/functional tests for datastore.

This commit only tests
- Adding a basic entity with an auto allocated ID, a
  given integer ID or a given name.
- Adding multiple entities via a transaction
- The given kind has no other entities (i.e. no false positives)

This commit adds
- A sample file to set environment variables for a test runner.
- A test runner.
- A utility for getting test relevant environ variables.