add logic to calculate efficient reset

Based on the formula from Cookie Clicker's subreddit, we calculate the
number of heavenly chips we need to earn before resetting becomes more
efficient than not. This is done by the formula

E*T - C = 1/2 h^2

where:
E = current effective cookies per second divided by 1 trillion
T = seconds since session started divided by 1 trillion
C = total cookies earned this session
h = number of heavenly chips you will *gain* on reset

equivalently, we can re-write this formula to find by as:

h = sqrt(2*E*T - C*2)

This tells us how many heavenly chips we need to earn by resetting in
order to be more efficient to reset. Then, we subtract from this the total
number of chips we've already acquired. Thus we can display how many
heavenly chips remain to earn before an efficient reset.

Possible future enhancements would be to tie resets into the "purchase"
equation and automatically reset. However, I was not that ambitious yet.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>

Conflicts:
	fc_button.js