/sensors.json, self-documenting, build script

I added 3 things:

There is a /help url that documents the http routes you can use. It's linked to from the main html output.

/sensors.json includes the state of all the sensors as json. It should look like this, without the comments:

``` js
{
  "heat": true, // boolean of whether the heater is turned on
  "temperature": {
    "celsius": 40, // water temperature in degrees celsius
    "fahrenheit": 104, // water temperature in degrees fahrenheit
  },
  "jets": 0 // jet time remaining in milliseconds or 0 if the jets are off
}
```

There is a build script at `make.sh` although I get build errors that seem like the chip I am trying to flash doesn't have enough RAM. It could be something else.