Create v3_dynamic certificate renderer.

Adds a new certificate renderer, v3_dynamic, which includes some of the improvements from previous Stanford renderers and is intentended to obsolete them:
- Rolling certificate date calculation, with pretty printing
- Dynamic text font selection for optimal aesthetics when different unicode sets are used
- Dynamic text sizing that tries to make names and course titles as large as possible
- Nice dynamic centering
- Also, introduces new semantics for 'interstitial texts' to allow them to be used not only to note distinction, but also the conditions for which a particular cert was granted
- Makes interstitial texts into a defaultdict which should make it safe to reference them everywhere without lots of annoying .get() failover
- Adding a test case in Travis for v3_dynamic certs with blank PDF template