Use standard ivy layout pattern in ivy support

Gradle's ivy-publish plugin uses a non-standard ivy layout pattern. I found a [mailing list post](http://gradle.1045684.n5.nabble.com/Standard-patterns-for-ivy-repositories-td4793222.html) where this feature was implemented and the developer asked if anyone knew if there was a standard layout. They weren't sure what the default ivy layout was and so used the maven layout. However, there is indeed a default ivy layout and it is different than the Maven layout. The fact that Gradle does not properly implement this makes it very difficult to interoperate with other tools. I was able to fix it by specificying the layout as shown below. I would love to see Gradle's non-standard default be updated to the standard layout so that it will work with other tools.

    publishing {
      publications {
        ivyJava(IvyPublication) {
          from components.java
        }
      }
      repositories {
        ivy {
          url "${System.properties['user.home']}/.ivy2/local"
          layout 'pattern', {
            artifact '[organisation]/[module]/[revision]/[ext]s/[artifact](.[ext])'
            ivy '[organisation]/[module]/[revision]/[artifact]/[artifact](.[ext])'
          }
        }
      }
    }

You can see this is what is defined in [ivysettings-local.xml](http://grepcode.com/file/repository.springsource.com/org.apache.ant/com.springsource.org.apache.ivy/2.1.0/org/apache/ivy/core/settings/ivysettings-local.xml) except ivy has a [type] which gradle does not support so I had to replace it with [ext] and [artifact] in order to get the same result.

    <property name="ivy.local.default.ivy.pattern"      value="[organisation]/[module]/[revision]/[type]s/[artifact].[ext]" override="false"/>
    <property name="ivy.local.default.artifact.pattern" value="[organisation]/[module]/[revision]/[type]s/[artifact].[ext]" override="false"/>