- Added Specific exceptions
- str() instead of concatenation
- removed zone not found warning

Transip sdk also supports a private_key_file,
so forwarding that option to the provider.
Could be handy in combination with k8s secrets.

TransIP allows '@' as value to alias the root record.
'@' was on populate appended with the zone, which trigger an unneeded update.
'@' => '@.example.com.' -> 'example.com'
This fix will stop the unneeded update

- added unit tests for apply functions
- added another json config file for this
- 100% code coverage reached

- added test cases for test_apply, testing for basic create, update, delete records
- added previous state json file in test/fixtures to do the above

- added test cases for testin populate()
- added mock request responses in tests/fixtures/
- updated requirements.txt to require edgegrid-python

That's all Dyn supports and it's cleaner to match it than to scale dyn since
we'd lose precision we can't get back during populate.

Also adds thorough tests

We hit this bug internally. It generally needed to be fix and was a larger
potential problem than expected since it was assuming TD assocaitions rather
than looking at them directly. Test changes were a little involved to suss
out, otherwise this is a fairly clean and simple fix.

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>