Made internal functions more 'private'
Removed enum, mostly for performance purposes
More tests
get_region -> _get_or_pop_region, for documentation cleanness
Added Simfile.pop() / .pop_string()
Fixed tests
Simplified behavior of .get() / .get_chart() / etc

* .get() / .get_string() / .pop() / .pop_string() / .get_chart() now take
  an optional 'index' parameter that defaults to 0 instead of None
* As a result, those methods now return the first match if 'index' is not
  provided, rather than throwing an exception
* .get('NOTES') now raises ValueError instead of MultiInstanceError
* MultiInstanceError has been removed
v0.7.0