Email regex does not allow ampersand (&) in the username

[zadok3141]

https://github.com/plone/plone.schema/blob/master/plone/schema/email.py#L10

Similar to two other issues
#9
#6
the email regex does not allow ampersand in the username part of the email. Ampersand is valid according to Section 3.2.4 of RFC 2822.
The solution ewohnlich suggest in 6 sounds right.

[mauritsvanrees]

I have created a PR that should fix this and other issues: #40.

[gyst]

Fixed in #40