Ignore files created by maven-release-plugin

Maven Release Plugin provides a standard mechanism to release project 
artifacts.

During the various phases of the execution the plugin create various 
files, especially if you do a dry run that allows you to see the changes
that the plugin will perform on your pom files

	mvn release:prepare -D dryRun=true

It will create

- `release.properties` - copy of the properties to be used
- `pom.xml.releaseBackup` - the pom.xml prior to any changes
- `pom.xml.tag` - the pom.xml as it will look when tagged
- `pom.xml.next` - the pom.xml as it will look for the next iteration
Merge pull request #856 from oschrenk/patch-3

Ignore files created by maven-release-plugin