Define a non-S3 input registry.

Revises the approach taken in #233.

!! No longer casts NULLs in incoming JSON to NA.
Update inputHandler docs, revamped tests.
Removed deprecated function documentation.
Added tests, stop converting NULLs to NA.
Merge pull request #369 from trestletech/input-handler-patch

Define a non-S3 input registry.