Create export directory if it doesn't exist

Arquillian supports exporting the archive to a directory when the test is run using the following configuration:

<engine>
<deploymentExportPath>/tmp/</deploymentExportPath>
</engine>

However, if this directory doesn't exist, Arquillian fails. Instead, Arquillian should attempt to create the directory (and any parent directory).

Also, the user should not be required to add a trailing slash. Be sure to add a test to verify that it's not required.