launch different chrome path in tests.

add support for specifing the path to the google-chrome executable via provess.env.CHROME.
- allows test to be run on linux. etc
updated load-and-launch-app to use absolute path to extension.
- chrome on linux was not loading the extension with relative path despite cwd being set correctly
  this could be related to my specific version of chrome and not the platform at all but change this works in all cases.
Merge pull request #18 from soldair/master

launch custom chrome path in tests. launch and load absolute path