Added Validation option us_phone_number to validation class

This will remove all non numeric characters, count the total digits and compare it to the passed length argument (default is 10).

This allows US phone numbers such as:
(000) 000-0000
000-000-0000
0000000000
000 000 0000
or any other format that includes the number of digits required.
  
Note: Its up to the programmer to format the number after the info has been submitted and before it is saved.