Added test for retrieving multiple artifacts at a time (see #110)
We now mark files that we couldn't find in the cache as `.missing` (see #110)
We now write the list of remote repositories that were tried while locating the artifact (#110)
We also only write the `.missing` file if there were remote repositories.
We now call `artifactNotFound()` for all artifact types that were not found (#110)
Let's not check for any `.missing` marker files if the module node doesn't even exist (#110)
Added an extra test for `CEYLON_CODE`