Add ability to add email to partner history manually.

The existing OpenERP email functionality assumes that emails will go through OpenERP and limits the email history on a partner to read only. The use case should also be expanded to support the ability for users to add email history manually - if sent from non-supported clients or for security reasons their environment doesn't support smtp. The system has the email one2many field set to readonly=True. readonly should be removed on this field and allow users to enter emails manually - just like phone calls and meetings. The existing use case is shortsighted.