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. You can see the default ivy layout 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"/>

The fact that Gradle does not properly implement this makes it very difficult to interoperate with other tools. I can fix it by specifying my own layout in my build file, but I've been frustrated having to specify this in all of my build files in order to get Gradle to work properly with other tools.