Support token-based auth

[jeffgus]

Instead of putting plain passwords in the settings.xml file, it would be nice just to generate a token and use that in the settings.xml. e.g.:

    <server>
      <id>internal-releases</id>
      <configuration>
        <httpHeaders>
          <property>
            <name>Authorization</name>
            <value>token blah</value>
          </property>
        </httpHeaders>
      </configuration>
    </server>