Add support for HTTP POST encryption ( using JWT )

[skyghost2210]

Added a new type of alert ( extend from HTTP POST ). This type will encrypt JSON alert using JWT ( JSON Web Tokens ).

Example of useage:

alert: jwt_post
http_post_jwt_key: "secret"
http_post_jwt_algorithm: "HS256"
http_post_url: "url_here"

[nsano-rururu]

You need to add "PyJWT==1.7.1" To requirements.txt. Because running docker test uses requirements.txt instead of setup.py (see Docker-test file)

[skyghost2210]

I have added "PyJWT==1.7.1" to requirements.txt. Can you check again? Thank you

[nsano-rururu]

I am getting the following error.
You need to follow the fork / clone repository on GitHub to the original repository.

Command execution example

git remote add upstream git://github.com/Yelp/elastalert.git
git fetch upstream

git merge upstream/master
git push origin master

[skyghost2210]

I have resolved all the conflicts. Can you check again? Thank you