Fixup JvmRun  only-write-cmd-line flag to accept relative paths.

Introduce expand_path fs utility and use it on JvmRun.
Add tests for this new util and JvmRun's cmdline-only mode.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/tasks/:jvm_run tests/python/pants_test/fs --test-pytest-options="-v -k'test_jvm_run or test_expand_path'"
...
11:10:49 00:00   [test]
11:10:49 00:00     [pytest]
11:10:49 00:00       [run]
                     ============== test session starts ===============
                     platform linux2 -- Python 2.6.9 -- py-1.4.23 -- pytest-2.6.1 -- /usr/bin/python2.6
                     plugins: cov, timeout
                     collected 12 items

                     tests/python/pants_test/fs/test_expand_path.py::ExpandPathTest::test_absolute PASSED
                     tests/python/pants_test/fs/test_expand_path.py::ExpandPathTest::test_dot_relative PASSED
                     tests/python/pants_test/fs/test_expand_path.py::ExpandPathTest::test_env_var_expansion PASSED
                     tests/python/pants_test/fs/test_expand_path.py::ExpandPathTest::test_pure_relative PASSED
                     tests/python/pants_test/fs/test_expand_path.py::ExpandPathTest::test_user_expansion PASSED
                     tests/python/pants_test/tasks/test_jvm_run.py::JvmRunTest::test_cmdline_only PASSED

                      6 tests deselected by '-ktest_jvm_run or test_expand_path'
                     ===== 6 passed, 6 deselected in 0.19 seconds =====

11:10:50 00:01     [junit]
11:10:50 00:01     [specs]
               SUCCESS

Bugs closed: 494, 495

Reviewed at https://rbcommons.com/s/twitter/r/908/