fixed importing of 'utils' in 'slate.classes' so that it wouldn't clobber when users have another module called 'utils'

[violet4]

• I kept it simple for ease of review and merging
• my friend had a namespace clobber - slate's insert utils imported a different utils
• I don't know best practices or whether import slate.utils as utils is the best solution, but using such an import never caused issues for me. from . import utils can cause issues when trying to run a code file directly (e.g. python classes.py from the command line) vs importing a module that contains such from . import utils statements from other python code.

[violet4]

I see a better solution in #32 from slate import utils described by https://github.com/timClicks/slate/pull/32#issuecomment-224019698