add nexus module

This adds a module for pulling artifacts from a Nexus repository. 

An example use case for deploying a snapshot artifact into Tomcat:
```
- name: Ensure foo application in container
  nexus:  server=nexus.example.com
          repository=example
          group=com.example.foo
          artifact=foo
          version=3.4.7-SNAPSHOT
          extension=war
          dest=/var/lib/tomcat6/webapps/foo.war
```
Is there a reason you use /nexus at the start of the path? We deploy our nexus at root context, so this URL won't work there