Create SecureHeadersMiddleware.py

[vaibhavmule]

Closes #502

[coveralls]

Coverage increased (+0.02%) to 89.555% when pulling 0838bb3 on vaibhavmule-secure-headers into f56944a on 2.1.

[josephmancuso]

Also need to add tests. Since you made a branch from this repo I can add them tonight if I get around to it. Test really would be just passing in a request object, running the after method and then asserting some of the headers were set correctly.

[vaibhavmule]

@josephmancuso I wasn't sure about branch that's why I choose 2.1

[josephmancuso]

@vaibhavmule this is important to note. Since we are adding it in this file, we can't import it or we will get a circular import. so we pass it in this after method instead to lazy load it. Not doing this was causing the middleware import to fail