Submit event

> *Important*: Please review and merge #117 first; this PR builds on those changes because composer support is essential

> *Important*: Do NOT open the changes tab or commit #b7e0749 in Github; this commit contains the committed vendor folder and will probably crash your browser or make it unusable.

This commit is intended to add support for submitting new events to the Joind.in API. In order to properly validate, render and secure forms I have chosen to include Symfony Forms and had to construct middleware in order to connect Slim to the Symfony components.