rename run_postprocessors and run_preprocessors

If validator class needs to perform some post- or preprocessing it should
inherit from Validator's class preprocess and/or postprocess methods instead of
registering its own members using add_(pre|post)processor.
use postprocess in List validator

Inheriting from process() and calling super() is equivalent.
remove method process() and leave __call__ only

Adding preprocess() and postprocess() methods and calling it from process()
would be a bit misleading. And now there is only one way to process value - by
calling a validator.
when binding validators add pre- and postprocessors using add_(pre|post)processor methods
ignore AttributeError when read-only value property is set

This allows to have read-only @property decorated value that contains some sort
of proxy object. Since the value can now be customized in that way, currently
checking if conversion was successful must be done by checking return value of
Validator's __call__ method (if the value was None, conversion failed)
Map validator changes

* raw_value is set only if validator is called to process value
* validator is valid if and only if all nested validators are valid
fix validity checking in Map validator

Now Map validator is invalid if it contains error or if one of nested
validators is invalid.
remove strict_processing from Map validator

Like in Schema, giving invalid key should result in exception being thrown and
this behaviour should not be disabled by any flag
allow sequence of keys as options for BaseChoice validator
fix AttributeError being raised when default specified for Choice validator