Fixes a regression on sub module loading

CasperJS didn't find any more users modules, when we
indicate only the name of the main script (it worked only
when we indicate the full path of the script on the command line).

Fixes #601
Cli tests: adds the casperjs output in command errors
clitests: Fixes issue with phantomjs path

In travis environment, PHANTOMJS_EXECUTABLE contains a relative
path to phantomjs executable.

However, new tests change the working directory, so this path becomes
invalid. We should set PHANTOMJS_EXECUTABLE to an absolute path.
Merge pull request #609 from laurentj/issue-601

Fixes a regression on sub module loading