Decide the fate of the MessegeBox property

[Szayet]

The documentation shows the MessageBox both being as a global/constant property and as an instanced property.

Pro for constant:

• Would solve MesseBox issue #3 use-case scenario.

Contra:

• Could not create Schema specific errors.

Also worth considering the MesseBox related ehancment plan #53.

[aldeed]

I don't think there's anything to do here. messageBox is instance-specific, but you can call SimpleSchema.setDefaultMessages to set the defaults used by all schemas initially.