Validators

* Handle None values gracefully in domain validator (144, pull request courtesy reahaas)
* Local part of the email address should be less or equal than 64 bytes (147, pull request courtesy mondeja)
* Removed py27 support
* Removed pypy2 support

* Made domain validation case-insensitive (136, pull request courtesy ehmkah)

* Updated domain validator regex to not allow numeric only TLDs (133, pull request courtesy jmeridth)
* Allow for idna encoded domains (133, pull request courtesy jmeridth)

* Added new validators `ipv4_cidr`, `ipv6_cidr` (117, pull request courtesy woodruffw)

* Added new validator: `es_doi`, `es_nif`, `es_cif`, `es_nie` (121, pull request courtesy kingbuzzman)

* Fixed domain validator for single character domains (118, pull request courtesy kingbuzzman)