Refactored into separate findPath and walk functions

I'm open to suggestions to changes, but basically the functionality in `bot.navigate.to` has been split into two functions (`bot.navigate.findPath` and `bot.navigate.walk`). `bot.navigate.to` still exists, but uses `findPath` and `walk`. 

Other:
* Updated docs
* Made `tooFarThreshold` configurable
* Fixed bug where bot likes to jump down steep hills