Add a separate field to hold Phone Extensions

Add a new field for Phone Extensions. Separating extensions from main phone number makes it easier to implement phone number format validations and also facilitates integration with automated dialing applications.

1. Add phone_ext VARCHAR 16 to Phone.xml (civicrm_phone table)
2. Add UI for editing extension to contact edit, and display to contact summary
3. Allow export of phone extensions
4. Handle upgrade for 3.3.7 and for 3.4.2 (for 3.4.2 have to add column if it's not already there)
5. Write script to clean up existing phone data, moving extensions into the new phone_ext column

Currently on hold:
6. Support Import of phone extensions
7. Allow use of phone extension in profiles (low priority)