Make flasgger dep optional (#26)

[AdamWill]

As discussed in the ticket, the flasgger dep is a pretty heavy one which is not needed when using httpbin as a library. It's only really needed to produce the fancy homepage and API docs for httpbin.org.

This makes the dependency optional, and falls back to the old static HTML page for / if flasgger is not available. The flasgger dependency is moved from the main set of dependencies to the 'mainapp' extras (to ensure we do get the shiny new homepage when we want it).