DataBlue: /poll request can return invalid JSON

A request to the datablue /poll endpoint can return invalid JSON in some error conditions.

For example, if the value of a query parameter cannot be parsed as a float value:
```https://data.cloudblue.org/poll?ma=4C:75:25:03:F8:8C&dk=76267409&A0=12a```
This will return the following error:
```{"er":"invalid value"}{"ma":"4C:75:25:03:F8:8C","rc":1,"vs":-617553116}```
Which is not valid JSON.

We should convert datablue to use std library json marshalling for responses to prevent errors like this and make the handlers more readable and configurable.