Add a shortcut method for rendering a block with it's own runtime
Make context optional
Merge pull request #110 from cpennington/shortcut-render

Add a shortcut method for rendering a block with its own runtime
Replace the timedelta test.

The timedelta test covered a specific issue with fields that didn't
round-trip their JSON identically.  That pulled in more code from
edx-platform than we'd like for a test.  This test shows the same
behavior, but in a much smaller test.

BTW: I don't think the field data machinery is still susceptible to this
problem, because it doesn't rely on json as it used to, but it's good to
keep the test cases we have.
Merge pull request #111 from edx/ned/remove-timedelta-test

Replace the timedelta test.
Fix bug in which an HTML block with only text body was not being loaded as unicode, and so was failing assert in Fragment.
Fix bug where XML imported blocks did not set their parents.