Allow passing a list of service types to discover()

diff --git a/discovery.py b/discovery.py
index 30376ee..d20386e 100644
--- a/discovery.py
+++ b/discovery.py
@@ -111,12 +111,12 @@ class ZeroconfListener(object):
         services[sname] = svc
 #         print "services[%s] = %s" % (sname, svc)

-def discover(timeout=None):
+def discover(services=('_ssh._tcp','_workstation._tcp','_nameserver._udp'),timeout=None):
     global browser
     lock = threading.Lock()
     lock.acquire()
     listener = ZeroconfListener()
-    browser = Browser(args=('_ssh._tcp','_workstation._tcp','_nameserver._udp'))
+    browser = Browser(args=services)
     browser.timeout = timeout
     browser.lock = lock
     browser.set_listener(listener)