Ensure device serials are properly sanitized for file system use.

Closes #140.

@edenman @holmes 

```
$ find spoon-sample/tests/target/spoon-output -type d
spoon-sample/tests/target/spoon-output
spoon-sample/tests/target/spoon-output/device
spoon-sample/tests/target/spoon-output/image
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.LoginActivityTest
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.LoginActivityTest/testBlankPassword_ShowsError
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.LoginActivityTest/testBlankUsername_ShowsError
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.LoginActivityTest/testEmptyForm_ShowsBothErrors
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.LoginActivityTest/testPasswordTooShort_ShowsError
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.LoginActivityTest/testValidValues_StartsNewActivity
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.OrderActivityTest
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.OrderActivityTest/testMakeASandwich_ItTastesGood
spoon-sample/tests/target/spoon-output/image/192_168_56_101_5555/com.example.spoon.ordering.tests.OrderActivityTest/testMakeSomeSalad_ItIsHealthy
spoon-sample/tests/target/spoon-output/junit-reports
spoon-sample/tests/target/spoon-output/logs
spoon-sample/tests/target/spoon-output/logs/192_168_56_101_5555
spoon-sample/tests/target/spoon-output/logs/192_168_56_101_5555/com.example.spoon.ordering.tests.LoginActivityTest
spoon-sample/tests/target/spoon-output/logs/192_168_56_101_5555/com.example.spoon.ordering.tests.MiscellaneousTest
spoon-sample/tests/target/spoon-output/logs/192_168_56_101_5555/com.example.spoon.ordering.tests.OrderActivityTest
spoon-sample/tests/target/spoon-output/static
spoon-sample/tests/target/spoon-output/test
spoon-sample/tests/target/spoon-output/test/com.example.spoon.ordering.tests.LoginActivityTest
spoon-sample/tests/target/spoon-output/test/com.example.spoon.ordering.tests.MiscellaneousTest
spoon-sample/tests/target/spoon-output/test/com.example.spoon.ordering.tests.OrderActivityTest
spoon-sample/tests/target/spoon-output/work
spoon-sample/tests/target/spoon-output/work/192_168_56_101_5555
```
```
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Spoon (Parent) .................................... SUCCESS [0.710s]
[INFO] Spoon Client Library .............................. SUCCESS [2.776s]
[INFO] Spoon Runner ...................................... SUCCESS [4.634s]
[INFO] Spoon Maven Plugin ................................ SUCCESS [2.841s]
[INFO] Spoon Sample (Parent) ............................. SUCCESS [0.004s]
[INFO] Spoon Sample Application .......................... SUCCESS [22.433s]
[INFO] Spoon Sample Instrumentation ...................... SUCCESS [1:29.404s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2:03.609s
[INFO] Finished at: Thu Oct 24 10:59:14 PDT 2013
[INFO] Final Memory: 59M/830M
[INFO] ------------------------------------------------------------------------
```