Create Checker class

Each implementation of this class will perform a different type of
health check.

Tested:
$ ./health-scan gBMCExample.pb
Protobuf model for machine family: Example
Verified that model matches current hardware

Signed-off-by: Benjamin Fair <benjaminfair@google.com>
Change-Id: I360539f913cfc659fc658b949a3a0f1ce102e5e3