Fixes #123. Support out-of-tree maven builds.

[tomaswolf]

Don't try to shorten paths to xsd sources. XCJ is called directly, not via the command-line, so doing so doesn't really help. Not trying to shorten these paths also avoids stripping a leading '/', which would otherwise make XCJ treat the path as relative and resolve to the current directory, which breaks out-of-tree builds.

Adds an integration test that creates a maven project in the system tmp directory, runs a maven build there, and verifies that that maven build was successful and did generate something.

[slachiewicz]

It's already fixed in 3.1.0 - I'll reuse the integration test. thx!