Implements a generic mechanism to avoid slow tests if $NOSE_EVAL_ATTR is set.

Allows the environment variable $NOSE_EVAL_ATTR to indicate which tests to skip during conda build (c.f. bob/bob.nightlies#55)

@tiago.pereira: all CI recipes will now transparently respect the value of $NOSE_EVAL_ATTR. The same mechanism also works locally, for building/testing packages.

merged

mentioned in commit b4c7c9c1

wow, thanks @andre.anjos

i will revert the patches i made it in some recipes triggering the previous version of this feature.