Allow to pass error-handlers to a `Validator`-instance

this is a follow-up to #89 and #90.

i propose to introduce error-handlers in order to allow to deal with errors more flexible.

to achieve that:

• Validator.__init__ takes an optional error_handler-object
• Validator._error is extended, so it stores the following data about an error:
  • trail - a list that represents the path to the field in the document (eg: ['a_dict', 'a_list']); a common prefix can be specified upon calling Validator.validate
  • field, value - as of now
  • constraint - the constraint that failed
  • message - a simple error message, like currently implemented
• Validator.errors calls the format(?)-method of error_handler that may return errors in a desired format and / or do whatever its purpose is

there will be one default-handler and two as reference-implementations:

• BasicErrorHandler
  • returns errors as now
  • but concentates trail and field
• HumanReadableErrorHandler
  • is targeted to end-users
  • concentates trail and field
  • suggests valid keys, in case of an unallowed value
  • the index of list-items is increased by one
  • list-items are prefixed with item #
• YamlErrorHandler
  • structures errors in a dictionary
  • joins it into a yaml-file

[prudhvee]

Is HumanReadableErrorHandler implemented? Couldn’t find the documentation.

[funkyfuture]

closing this as the core interface is now available.