Better EMR coverage and boto3 request/response handling

This revision includes:

- A handler for requests for which content-type is JSON (from boto3).

- A decorator (generate_boto3_response) to convert XML responses to
  JSON (for boto3). This way, existing response templates for boto can
  be shared for generating boto3 response.

- Utility class/functions to use botocore's service specification data
  (accessible under botocore.data) for type casting, from query
  parameters to Python objects and XML to JSON.

- Updates to response handlers/models to cover more EMR end points and
  mockable parameters

setup.py

@@ -10,6 +10,7 @@ install_requires = [
     "xmltodict",
     "six",
     "werkzeug",
+    "pytz"
 ]
 
 extras_require = {