cmd/camget, cmd/camput, pkg/schema: Support FIFOs.

This patch:

1. Introduces a JSON schema for FIFOs (named pipes).
2. Adds support for uploading and restoring FIFOs to camput and camget
respectively.

It includes tests for the basic schema support in pkg/schema and
integration tests for camput and camget.

This CL does not add FIFO support to the HTTP server and fs (cammount
support) code. Those will be addressed in a subsequent patch.

Change-Id: Iab926073c36e653d25e693a559178bea9bf19b9a
Merge branch 'fifo-support' into toshnix

Conflicts:
	pkg/schema/schema_test.go

Change-Id: I60ec487832c85b3c46efdb5b863344f4af3d5a69
README: Note FIFO support.

Change-Id: Id6410ce579008c6835ff6c14189f0c93a927fd0f