run tests in multiple envronments with different configuration

[elf-pavlik]

TravisCI docs: Set environment variables
test helper introduced in a008aef should make it simpler!

• dev in memory with node.js
• pre-commit leveldown with node.js
• browser with browserifying on the fly
• browser using sandalone files in ./dist/ (+ bower)
• all above with addition of sublevel
• browser with level-js and mem
• node.js with multilevel

[mcollina]

Can we please discuss this in LevelGraph and then updated the others?
LevelGraph is more tricky in any case, the other should just work.

[elf-pavlik]

sure! i just wanted make sure not to loose track on task to setup testing versions we make available through bower!