Labels and milestones

**Summary**
Added a script to add or update repo milestones. Milestone script requires content in all fields, even though only title is required by the API. (Script throws a null value error when optional fields are left empty.)

Fixed a bug in add-standard-labels.py where spaces were not quoted properly, causing a fatal error on label update.

**Testing**
add-standard-labels.py:
1. Run add-standard-labels.py against a test repo using the script defaults. Labels should now be available on the repo's issues page.
2. Create a number of dummy issues and apply labels to some (not all).
3. Add a new label to the list of defaults in add-standard-labels.py and change the label color for an existing label.
4. Rerun the script. The new label should be available and the updated label should reflect the correct color. Issues created in step 2 should be labelled correctly.

add-standard-milestones.py:
Repeat label tests using milestones. Note that all fields are required in the script even though most are optional in the api.