Add support for tox (http://tox.testrun.org/)

Sample tox output (on an OS X 10.6.8 system):

```
[last: 0] marca@scml-marca:~/dev/git-repos/python-dtrace$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/python-dtrace/setup.py
py26 sdist-reinst: /Users/marca/dev/git-repos/python-dtrace/.tox/dist/python-dtrace-0.0.5.zip
py26 runtests: commands[0]
WARNING:test command found but not installed in testenv
  cmd: /usr/bin/sudo
  env: /Users/marca/dev/git-repos/python-dtrace/.tox/py26
Maybe forgot to specify a dependency?
1793 1 [64696, 'Google Chrome H'] 1
1793 1 [73973, 'Google Chrome H'] 1
1793 1 [75215, 'Google Chrome H'] 1
1793 1 [38, 'cupsd'] 2
1793 1 [457, 'postgres'] 2
1793 1 [64662, 'CloudRaindropHe'] 2
1793 1 [64678, 'Google Chrome H'] 3
1793 1 [64700, 'Google Chrome H'] 3
1793 1 [73890, 'Google Chrome H'] 3
1793 1 [456, 'postgres'] 4
1793 1 [371, 'GrowlHelperApp'] 5
1793 1 [379, 'Cloud'] 5
1793 1 [8673, 'mdworker'] 5
1793 1 [281, 'Dock'] 6
1793 1 [285, 'fontd'] 6
1793 1 [365, 'mysqld'] 6
1793 1 [64726, 'Google Chrome H'] 6
1793 1 [55445, 'DashboardClient'] 7
1793 1 [97405, 'AppleScript Edi'] 7
1793 1 [30, 'coreservicesd'] 8
1793 1 [110, 'WindowServer'] 8
1793 1 [52, 'mds'] 10
1793 1 [21, 'configd'] 12
1793 1 [108, 'nmbd'] 13
1793 1 [37, 'ntpd'] 14
1793 1 [64705, 'Google Chrome H'] 18
1793 1 [302, 'redis-server'] 20
1793 1 [454, 'postgres'] 20
1793 1 [455, 'postgres'] 20
1793 1 [25, 'mDNSResponder'] 22
1793 1 [283, 'Finder'] 23
1793 1 [76179, 'python'] 28
1793 1 [63377, 'plugin-containe'] 54
1793 1 [58, 'fseventsd'] 62
1793 1 [60313, 'natd'] 81
1793 1 [57691, 'Dropbox'] 87
1793 1 [282, 'SystemUIServer'] 89
1793 1 [64661, 'Google Chrome'] 132
1793 1 [498, 'iTerm'] 242
1793 1 [89177, 'firefox'] 355
1793 1 [303, 'mongod'] 458
1793 1 [64708, 'Google Chrome H'] 734
py27 sdist-reinst: /Users/marca/dev/git-repos/python-dtrace/.tox/dist/python-dtrace-0.0.5.zip
py27 runtests: commands[0]
WARNING:test command found but not installed in testenv
  cmd: /usr/bin/sudo
  env: /Users/marca/dev/git-repos/python-dtrace/.tox/py27
Maybe forgot to specify a dependency?
1793 1 [64697, 'Google Chrome H'] 1
1793 1 [64705, 'Google Chrome H'] 1
1793 1 [73371, 'Google Chrome H'] 1
1793 1 [73890, 'Google Chrome H'] 1
1793 1 [73973, 'Google Chrome H'] 1
1793 1 [38, 'cupsd'] 2
1793 1 [293, 'ARDAgent'] 2
1793 1 [457, 'postgres'] 2
1793 1 [37948, 'usbmuxd'] 2
1793 1 [55445, 'DashboardClient'] 2
1793 1 [97405, 'AppleScript Edi'] 2
1793 1 [64700, 'Google Chrome H'] 3
1793 1 [456, 'postgres'] 4
1793 1 [12405, 'dbfseventsd'] 4
1793 1 [365, 'mysqld'] 5
1793 1 [371, 'GrowlHelperApp'] 5
1793 1 [373, 'Alfred'] 5
1793 1 [8673, 'mdworker'] 5
1793 1 [64678, 'Google Chrome H'] 5
1793 1 [17, 'kextd'] 6
1793 1 [30, 'coreservicesd'] 8
1793 1 [21, 'configd'] 12
1793 1 [37, 'ntpd'] 14
1793 1 [22, 'syslogd'] 15
1793 1 [3848, 'smbd'] 15
1793 1 [76205, 'launchd'] 16
1793 1 [110, 'WindowServer'] 19
1793 1 [302, 'redis-server'] 20
1793 1 [454, 'postgres'] 20
1793 1 [455, 'postgres'] 20
1793 1 [108, 'nmbd'] 25
1793 1 [76204, 'python'] 26
1793 1 [8674, 'mdworker'] 28
1793 1 [63377, 'plugin-containe'] 54
1793 1 [12409, 'dbfseventsd'] 55
1793 1 [58, 'fseventsd'] 71
1793 1 [57691, 'Dropbox'] 72
1793 1 [12408, 'dbfseventsd'] 79
1793 1 [282, 'SystemUIServer'] 84
1793 1 [25, 'mDNSResponder'] 95
1793 1 [64661, 'Google Chrome'] 100
1793 1 [269, 'launchd'] 125
1793 1 [60313, 'natd'] 126
1793 1 [76210, 'smbclient'] 166
1793 1 [52, 'mds'] 172
1793 1 [76206, 'nmblookup'] 180
1793 1 [76209, 'nmblookup'] 180
1793 1 [76207, 'smbclient'] 224
1793 1 [1, 'launchd'] 238
1793 1 [498, 'iTerm'] 242
1793 1 [76205, 'winbindd'] 313
1793 1 [18, 'DirectoryServic'] 322
1793 1 [283, 'Finder'] 389
1793 1 [303, 'mongod'] 461
1793 1 [76208, 'smbd'] 568
1793 1 [89177, 'firefox'] 600
1793 1 [64708, 'Google Chrome H'] 731
_________________________________________________________________________ summary __________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  congratulations :)
```