Implementing pattern cache with a maximum size of entries.
Adding transient maven xml file for radio to .gitignore
Adding stream link to stream alert mails if web if uri is configured.

The generated link shows the messages for the time range the alert is
based on.

Fixes #455
Adding configuration directive example for web if url.
Merge branch '0.20'

Conflicts:
	graylog2-server/src/main/java/org/graylog2/Configuration.java
	graylog2-server/src/main/java/org/graylog2/alerts/AlertSender.java