Removed ThreadScheduler as normal requirement

Some environments could have threading disabled, and therefore the threading import can resolve in an error.
This commit fixes that :)

promise/__init__.py

@@ -24,7 +24,11 @@
         get_default_scheduler,
         set_default_scheduler
     )
-    from .scheduler import SyncScheduler, ThreadScheduler
+    from .scheduler import SyncScheduler
+    try:
+        from .thread_sheduler import ThreadScheduler
+    except ImportError:
+        ThreadScheduler = None
 
     __all__ = [
         'Promise',

promise/scheduler.py

@@ -1,15 +1,6 @@
-from threading import Thread
-
-
 class SyncScheduler(object):
     def call(self, fn):
         try:
             fn()
         except:
             pass
-
-
-class ThreadScheduler(object):
-    def call(self, fn):
-        thread = Thread(target=fn)
-        thread.start()

promise/thread_scheduler.py

@@ -0,0 +1,7 @@
+from threading import Thread
+
+
+class ThreadScheduler(object):
+    def call(self, fn):
+        thread = Thread(target=fn)
+        thread.start()