fixes #23 to allow <env> to be used inside <configuration> in the pom.xml to specify env vars to add to the generated Dockerfile when using 'mvn docker:build'
added possible NPE fix
Merge branch 'env-var-fix' of github.com:jstrachan/docker-maven-plugin into jstrachan-env-var-fix

Conflicts:
	src/main/java/org/jolokia/docker/maven/AbstractDataImageSupportMojo.java
	src/main/java/org/jolokia/docker/maven/assembly/DockerArchiveCreator.java
	src/main/java/org/jolokia/docker/maven/assembly/DockerFileBuilder.java
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