Put the node version in the activate script

Means that people will not have to "nvm use 0.something" by hand; and,
documents in the activate script which node version we use.
source key.sh all the time: it is not optional
Add whitespace
Clean the test cleaning code
remove nvm use from README (it's in activate)
remove apt-get of npm as it's bundled with nvm
Add a note explaining that we use nvm
Merge pull request #572 from scraperwiki/improve-activate

Improve activate