Consolidate workaround checking into test_helper tests.

This will force the tests to fail if our workarounds ever become
redundant, so they can be removed as soon as possible.
Fix lack of single curly quotes in 'markdown' filter.

This is needed because of a bug in vmg/redcarpet#57. There is a test
assertion in test_helper to verify that this is actually necessary, and
the condition means it will automatically turn off should redcarpet have
this bug fixed for the user.
Update CHANGELOG.
Slightly better test: assert the negation.
Merge pull request #40 from aprescott/markdown/curly-quotes

Add a work-around to let the 'markdown' filter render curly single quotes properly