Add 'retrieve' function to instantiate a class and map it

[jacebrowning]

Additionally, yorm.store should simply call the mapper's store function if the object is already mapped.

[jacebrowning]

I'm thinking yorm.sync might work better (to do both).

[jacebrowning]

A few scenarios exist:

• yorm.sync is called on a unmapped object:
  1. apply mapping
  2. fetch from the file if it exists (and fetch=True)
  3. if the file doesn't exist, create the file
  4. store to the file (if store=True)
• yorm.sync is called on a mapped object:
  1. fetch from the file (and fetch=True)
  2. store to the file (if store=True)