Add email support

This commit adds a new starter to auto-configure a MailSender when the
necessary classes are present and when the property "spring.mail.host" is
set.

The auto-configuration also accepts any arbitrary properties that
JavaMail might need using the "spring.mail.properties" prefix.

Fixes gh-1760

spring-boot-starters/pom.xml

@@ -43,6 +43,7 @@
 		<module>spring-boot-starter-logging</module>
 		<module>spring-boot-starter-log4j</module>
 		<module>spring-boot-starter-log4j2</module>
+		<module>spring-boot-starter-mail</module>
 		<module>spring-boot-starter-mobile</module>
 		<module>spring-boot-starter-actuator</module>
 		<module>spring-boot-starter-parent</module>