renamed the exception class
imports cleanup

- also got the order correct for _activate and _resetContext
added context mgmt to Transform class

- allows for `with reset(), translate(), rotate(), skew(), scale():`
chaining (similar to the plotstyle commands)
- each command will reset not only its own state-modification, but
anything else transform-related that happens inside the block. (but is
this a bug or a feature?)
better behavior from transform context mgrs

- transform() will reset everything back to the way it was before the
block, erasing any changes made within.
- scale(), rotate(), etc. will undo their own modification at the end
of the block, but any other transformations applied within the block
will persist
- believe it or not this is actually handy