SlimerJS: fix listModulesInDir() handling of README.md files

Fixes #409 - fs.isFile was raising the following error in SlimerJS when
checking for files in not existing directories.

"Component returned failure code: 0x80520005 (NS_ERROR_FILE_DESTINATION_NOT_DIR) [nsILocalFile.isFile]"
Integration tests: allow options to be passed to phantomas

* test integration with SlimerJS (avoid regression like #409)
Merge pull request #410 from macbre/slimerjs-fs-list-fix

SlimerJS: phantomas dies on listing extensions directory