EIN tries to build a cached of pdef's for all potential completion symbols. This
is problematic because not all completion symbols have a pdef and can cause
python to throw exceptions in entertaining and hard to catch ways. We take a
liberal approach and ignore anything that throws an exception when we try to
inspect it.
Eldoc support should also be more robust because of these changes as well.
NBformat 4 has a tag key in the cell metadata. We use it to tag a cell
depending on which worksheet it is in. For now the code does nothing
else with this tag, but the goal is to reimplement multiple worksheets
in ein.
