FTP memory storage

This pull request changes the FTP integration test to no longer rely on an existing and writeable file system (think: XAR releases) but instead creates the file system in-memory during startup.

Test results:
```
$ unittest net.xp_framework.unittest.peer.ftp.IntegrationTest
[.........................]

OK: 25/25 run (0 skipped), 25 succeeded, 0 failed
Memory used: 2907.08 kB (3737.48 kB peak)
Time taken: 2.090 seconds
```