This splits the web/popminer/popminer.go file into 5 files:
 - dispatch.go: Contains all methods that can be dispatched from JS
 - network.go: Contains network settings
 - network_local.go: Contains localnet settings (behind build tag)
 - popminer.go: Contains the main function and core
 - util.go: Contains utilities for working with JS

This contains a lot of miscellaneous improvements, however the notable
ones are:
 - Invoke Promise resolve function with js.Value instead of marshaled
   JSON, meaning that the result can be used directly without the need
   to encode/decode JSON.

 - All errors use a common format containing the error message, a
   timestamp, and the debug stack, allowing for easier debugging.

 - Replace 'wasmPing' with 'version' - wasmPing is not very useful and
   was only used for testing the connection between JS and WASM. version
   returns useful version information and allows the same testing.

 - Add "Object" type that can be used to easily create JS Objects from
   Go. Calling Object.Value() will create a js.Value representing the
   object, converting each contained value to a js.Value. This also has
   the benefit of breaking the build if a key changes, preventing
   breaks in the JS API.

 - Move network configurations into a separate file and use constants. I
   think this is cleaner than the previous solution, and allows localnet
   to only be available when built with the 'hemi_localnet' build tag.