Switched to check for highest year in events for timeline length.

Previously used a variable in config.json but this makes the process
too manual. Added a function that takes a startYear and endYear,
preferably from a single time object, and checks to see if either
are higher than the current final year (which starts this year).

Still need to find a place to call it so that there aren't methods
with side-effects.
Merge branch 'issue-16' into gh-pages