Support batch email sending in "isolated recipient" mode

[niuxuetao]

Introduce <email> tag, for details, refer to the EmailTag class.

It supports smtp email sending with/without authentication, with/without ssl

It also supports sending separate emails based on the boolean @isolate attribute. when the value is true, it will not let any recipient know about any other recipient. This feature removes dependency on the well known batch bcc workaround.

Example: https://github.com/niuxuetao/paxml/blob/master/PaxmlCore/src/test/resources/selftest/emailTest.xml

[niuxuetao]

done in r 1.8