Introduce new switch for paver run_quality command.

When using -p, the underlying diff-quality command will pass that
number as a value for --fail-under.

For example,
  paver run_quality -p 75

Will pass 75 into the --fail-under switch on diff-quality.

Also upgrades diff-cover in order to take advantage of this funcitonality.
Handle OpenID errors for POST requests.

ECOM-441
Merge pull request #5552 from edx/benp/diff-cover-threshold2

Benp/diff cover threshold2
Enable parent/child tests using the acid XBlock.

Also fix those tests to correctly wait until the ajax portions have
finished before evaluating the page contents.
Add block.name to xblock javascript frontend
Add block type to the available attributes in the xblock javascript frontend
Set xmodule_runtime on all XModule-like things (including pure XBlocks using the XModuleMixin)

Expose xmodule_runtime as .runtime if it's set (otherwise, fall back to
the supplied runtime). This causes all blocks to act like XModules when
they have a ModuleSystem, and like XModuleDescriptors if they only have
a DescriptorSystem.
Escape xblock wrapper data attributes and css classes for safe html
Merge pull request #5594 from edx/diana/open-id-bug

Handle OpenID errors for POST requests.
Merge pull request #5590 from cpennington/xblocks-with-children

Enable pure XBlocks to use children in edx-platform
Do event tracking for major forum events.