Fix various sage import locations
Create nodoctest.py

This should fix the doctest errors in sagenb, as well as give a tiny bit of info to anyone seeking to use Grinder with sagenb.
Merge pull request #280 from jdemeyer/fix_imports

Fix various sage import locations
Disallow spellcheck in code cells
Document toggle between source code and doc

It was not clear before that it is a feature, not a bug, that after e.g.
`factorial?[tab]` if you tab again you get source code.  This is actually quite useful.
Merge pull request #283 from kcrisman/spellcheck

Disallow spellcheck in code cells
Use atomic_write() to have no corruption in case of crashes
Merge pull request #285 from kcrisman/togglesource

Document toggle between source code and doc
Fixes #275
Add doctest for pull request #287
Allow OSError
Merge pull request #282 from kcrisman/grindernodoctest

Create nodoctest.py  - fixes #281 about grinder tests
Remove unused load/save stuff
Clarify save and load of objects
Allow custom label in shortcut matrix creation in interact
Merge pull request #299 from kcrisman/matrixdefault

Allow custom label in shortcut matrix creation in interact
Merge pull request #292 from kcrisman/saveinfo

Clarify save and load of objects
Merge pull request #291 from kcrisman/loadsave

Remove unused load/save stuff
Merge pull request #287 from jdemeyer/atomic_write

Use atomic_write() to have no corruption in case of crashes