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-autoconfigure/pom.xml

@@ -380,6 +380,11 @@
 			<artifactId>jms-api</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>javax.mail</groupId>
+			<artifactId>mail</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.aspectj</groupId>
 			<artifactId>aspectjweaver</artifactId>