Re-raise IOErrors which happen inside configuration scripts.
Added a test for the bug where aspen wrongfully catches IOErrors that
occur in configuration scripts. I also added tests/_config_test.py, a
simple script that simple tries to open a nonexistant file. Previously
this would have raised a ConfigurationError saying that the config file
didn't exist, but now it correctly re-raises the IOError.
Use mk to create the config file dynamically instead of storing it in
the repo.
Merge pull request #213 from nejstastnejsistene/master

aspen wrongfully catches IOErrors that occur within configuration scripts