Make a base class for sentiment request handlers

We make a common base class for sentiment request handlers in order to
reuse code in the future. For example, our user facing query handler
will use much of the same code that our future API handler will need to
use as well.
Add: RESTful JSON API for sentiment analysis

Here we add the ability to programmatically request to classify a single
document (tweet) for sentiment analysis. This is progress towards gh-25.

Resource URL:
http://.../api/sentiment/service_name.json

Currently, we only accept GET requests, and 'classify' is the only valid
service name. In the future, we can add other services under different
names, such as a bulk classification service.
Separate image and URL link placement and code
Fix: GitHub rendering of Markdown image with link