Allow installation of ML free version.

Add an option to the install script, `--no-ml`, that when given will
install Talon without ML support.

Fixes #96

talon/__init__.py

@@ -1,7 +1,12 @@
 from talon.quotations import register_xpath_extensions
-from talon import signature
+try:
+    from talon import signature
+    ML_ENABLED = True
+except ImportError:
+    ML_ENABLED = False
 
 
 def init():
     register_xpath_extensions()
-    signature.initialize()
+    if ML_ENABLED:
+        signature.initialize()