Keep json numbers as strings in PayloadJsonDecoder

When a json number gets unmarshaled into an interface{}, it's interpreted
as a float64. When this gets converted back to a string (a few lines later),
it's forced to the format "%0.9f", which adds nine zeros after integer
values. Package encoding/json allows us to skip the two conversion steps,
which allows integers to remain integers.
Convert AMQPInput `decoders` option (supports multiple) to `decoder` (supports only
one).
Merge pull request #429 from rhysh/features/json-integers

Keep json numbers as strings in PayloadJsonDecoder
Merge pull request #433 from mozilla-services/features/amqp-single-decoder

AMQPInput supporting only a single decoder
Merge branch 'dev' into features/310-xmlpath