Re-enabling disabled slow tests.

Fixes #85.

Whatever issue was causing the slowdown was related to the
Py3k string/unicode issues. I'd guess it was in the crypto paths
where an expensive computation was being done for no reason.

Running them without network (to make sure mocks are correct):

- `py26openssl13` - Ran 152 tests in 6.163s
- `py26openssl14` - Ran 152 tests in 6.878s
- `py27openssl13` - Ran 152 tests in 4.657s
- `py27openssl14` - Ran 152 tests in 5.651s
- `py33openssl14` - Ran 152 tests in 6.685s
- `py34openssl14` - Ran 152 tests in 6.373s
- `pypyopenssl13` - Ran 152 tests in 4.932s
- `pypyopenssl14` - Ran 152 tests in 5.699s
Merge pull request #89 from dhermes/fix-slow-tests

Re-enabling disabled slow tests.
Supporting client secrets / user flow with a hack.
Adding a script and example creds file to check user implicit auth.

Also fixing bug where SETTINGS was immutable.