Huge Auto-Buy Update

Fixes:
* Save Settings button no longer starts an extra auto-buy timeout if one
   is already running.
* Auto-Buyer does an approximate BCI adjustment as it goes (fixes #57)

Improvements:
* Next Purchase timer accounts for Maintain Bank requirement (resolves #55)
* Changed Maintain Bank setting from checkbox to select
* New Maintain Bank option "Smart" will maintain a bank threshold as
   long as the banked cookies are more valuable (based on overall GC
   probabilities) than the building/upgrade that could be bought with them.
* Maintain Bank "Always" (formerly 'on' or checked) now maintains the
   Lucky threshold if Get Lucky has not been purchased or the
   Grandmatriarchs is Angered.  Previously, it wouldn't maintain anything
   until Get Lucky was purchased, and would maintain Lucky+Frenzy all the
   time after that.
* Auto-Buyer won't get tunnel vision when the most efficient purchase is
   very expensive.  It will buy a less efficient building/upgrade first if
   that one will pay for itself before the expensive one can be afforded.
Merge pull request #103 from super-aardvark/autobuy

Huge Auto-Buy Update