Clean up error definitions
Fix #58; add basic implementation of symbolic errors
Rename "sym_messages" to "symbolic"

That way you can do `outcome.errors.symbolic`.
Add test for #initialize
Renamed "sym_add" to "add_sym"
Implement #initialize_dup and #clear
Give names to tests
Fix return type syntax
Document ActiveInteraction::Errors
Call #add before adding symbolic errors

This means we can control the return type. Also, if `:strict` (in the
options) is true, it will raise before getting to our code.
Update changelog
Update internals to use add_sym
Update documentation