Add ability to override exception messages thrown in validation methods

[Atanas Angelov]

Currently, you can override just the main error message for a validation method but Exceptions provide a way to display error-specific messages which cannot be overridden.

Example syntax: field_name.validation_rule[.exception_code] ( [] optional )

Resulting use:

Carbon_Validator::register_default_error_message('file', 'Please upload a file in the "%1$s" field.');
Carbon_Validator::register_default_error_message('file.1', 'Please upload an image file for your avatar.');

$strings = array(
    'avatar.file'=>'Please upload a file in the "%1$s" field.',
    'avatar.file.1'=>'Please upload an image file for your avatar.',
);

In addition, it may be better for Carbon_Validator::register_default_error_message to accept a callback which returns a string as well.

[Atanas Angelov]

Abort in favor of not throwing exceptions in built-in validations and making required work with files instead (if it doesn't already).

Also, references to self:: are present in built-in validation methods which must be removed.

[html Burger]

Fixed in pull request #4