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.
added xmlpath dependency
added tests around PayloadXmlDecoder for issue #310
added documentation for PayloadXmlDecoder
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 branch 'features/amqp-single-decoder' into features/no-encoding-header
Merge pull request #433 from mozilla-services/features/amqp-single-decoder

AMQPInput supporting only a single decoder
Merge branch 'dev' into features/no-encoding-header
Remove message encoding header from Heka protocol.
Merge branch 'dev' into features/310-xmlpath
Merge pull request #430 from mozilla-services/features/310-xmlpath

adds a PayloadXmlDecoder
Merge pull request #434 from mozilla-services/features/no-encoding-header

Features/no encoding header
Add comment to clarify use of `a = b[n:n]` pattern.
Use a separate report recycle chan for the building of reports so that they can be created even if other things are jammed up.
Merge branch 'dev' into features/issue_423
Skip inject chan populating.
Merge pull request #435 from mozilla-services/features/issue_423

Features/issue 423
Add support for `cascade_strategy` config option to change the way we iterate
through the subs.
Add closing bracket to libc6 dependency declaration