Added support of maven plugins in project.clj.

It's possible to add definition :plugins in project definition.
Example: 
 :plugins [[com.theoryinpractise/clojure-maven-plugin "1.3.13"]
           [org.apache.tomcat.maven/tomcat7-maven-plugin "2.1"]]
Maven plugins are now supported by :pom-plugins to avoid name conflict
with Leiningen :plugins vector. 

Added maven <configuration/> tag inside of plugin definition.

Added usecase of :pom-plugins in sample.project.clj
Merge pull request #1324 from dominikdz/master

Added support of maven <plugin> in pom.xml (pom-plugin tag)