put the string 'now' instead of the current date in the querystring when it is today

[mpdehaan]

We tend to add the current date into the querystring so that when someone shares a bookmark, the user sees the same time range the person who linked it sees, forever.

This is well meaning, but isnt' always what someone wants to bookmark.

Rather, than doing this:

start=START_DATE_HERE&end=TODAYS_DATE

we should by default bookmark something like:

start=START_DATE_HERE&end=now

and the word "now" should be magic for "calculate today's date".

If the date is not specified in the querystring, the system should use "now" versus filling in the date, but if the user specifies the end date, it should be sticky.

Prereq for #72