Have the machine environment as a map.
Add a package for our environment variables.
New tests and some updates
Put the import in the wrong place.
- Some fixes per rog's suggestions
- Now fixed megawatcher with a properly failing test
- More review points
- Go fmt
- Fix review comments
[r=sidnei] Fix megawatcher to also unescape settings

Per rog's review, cleaned up a few things here and there and also fix
megawatcher to unescape properly.

https://codereview.appspot.com/12168043/

R=dimitern, rogpeppe
Merge trunk
[r=thumper] Use a map to store machine environment in config.

The MachineConfig now uses a map to store the environment
for the machine agent.

https://codereview.appspot.com/12005048/