add validators module

Provides new base class called Validator. This mostly serves as an
interface definition for now.

starcluster/__init__.py

@@ -14,6 +14,7 @@
     "plugins",
     "balancers",
     "managers",
+    "validators",
     "image",
     "volume",
     "awsutils",

starcluster/validators.py

@@ -0,0 +1,15 @@
+class Validator(object):
+    """
+    Base class for all validating classes
+    """
+    def validate(self):
+        """
+        Raises an exception if any validation tests fail
+        """
+        pass
+
+    def is_valid(self):
+        """
+        Returns False if any validation tests fail, otherwise returns True
+        """
+        pass