Support for non-relative output directories

This feature makes .classpath like following:

`````
<?xml version="1.0" encoding="UTF-8"?>
<classpath>
        <classpathentry kind="src" output="-mnt-tmpfs-okumura.y-workspace-sbt-DM-scala-2.10-classes" path="app"/>
        <classpathentry kind="src" output="-mnt-tmpfs-okumura.y-workspace-sbt-DM-scala-2.10-test-classes" path="test"/>
`````
And .project is like:

`````
                <link>
                        <name>-mnt-tmpfs-okumura.y-workspace-sbt-DM-scala-2.10-classes</name>
                        <type>2</type>
                        <location>/mnt/tmpfs/okumura.y/workspace/sbt/DM/scala-2.10/classes</location>
                </link>
                <link>
                        <name>-mnt-tmpfs-okumura.y-workspace-sbt-DM-scala-2.10-test-classes</name>
                        <type>2</type>
                        <location>/mnt/tmpfs/okumura.y/workspace/sbt/DM/scala-2.10/test-classes</location>
                </link>
`````
This allows to use non-relative target such as tmpfs and  I think this fix #128