Add support for units to Metric object
define units properly within the MySQL check
fix bug where nil could be passed as argument to fmt
add all known units to existing checks
modify sample check_metrics post request to include units and contain expected keys
verify that units present on serialized metrics ready for output
remove #TODOs and add units to memorycheck check tables
fix mysql check core.uptime metric unit to seconds, not milliseconds
Merge pull request #325 from racker/CMD/52/allow_units_on_agent_custom_checks

WIP -- Add support in agent for units attached to metrics