- use upstream retry_decorator as external dependency
  + expects: https://github.com/pnpnpn/retry-decorator
  + resolves issues #152
Make gsutil error text include <Message> content (fixes #154)
Add notification URL configuration for notification tests.
Fix unit test breakage because VERSION file is old.

Previous test relied on the modified time of the VERSION file being < 30 days
ago. I mocked out the modification time and added an additional test to test
both cases.
Add more detailed error message to notification watchbucket command.
Make gsutil cp not fail if unable to check versioning config on dest bucket
Move all TTY checks to a common util function and mock it for update tests.
Use transcoding-invariant headers when available in gs.Key
Enforce project_id entry in config command; provide friendly error if missing proj ID
Update Python version check and shebang line.

* Change python shebang line to python2 so systems where the default Python
  version is python3 will still work.
* Change python version check to make sure we're not running under python3.
* Add additional python version check to the beginning of the gsutil script
  since things can break before we get to the main() function.
Make gsutil pass bundled cacerts.txt to oauth2client; stop checking SHA1 of certs, now that we no longer depend on boto distribution
Fix recursive uploading from subdirectories with unexpanded wildcard as source URI (fixes #111).
* Fix handling of non-numeric version strings in update test.
Changed rm -r gs://bucket to delete bucket at end
Fix tests that perform operations on bucket listings.

The cors and lifecycle tests weren't using the Retry decorator like the ls test
was. Even with the ls test, it was possible that other invocations of the test
runner would collide with the bucket names being chosen for the operation. I
added a random prefix to reduce the chance of this happening and also added
the Retry decorator to the cors and lifecycle tests.
Change shebang line back to python because this doesn't work on some systems.
Implemented gsutil stat command
Fix broken rm tests; fix rm -ar logic and clarify error message
Updated gsutil help to point to Pantheon instead of older APIs console
Implement help for subcommands and add OPTIONS sections for subcommands