Added the ability to create a new milestone

This would resolve ticket #12

A new `Create new milestone` button has been added to the Milestones page. This will simply allow you to set the title of the new milestone and add it to the page.

For this pull request I decided to not include the changes to the `application.js` file. If you would like for me to add those changes, I can remove this PR and submit another one.

One thing I wanted to mention about the code is the placement of the `create_milestone` method in `lib/bridge/github/issues.rb`. I feel like it might be kind of weird to have it located in `issues.rb` so if you would rather have me create a `milestones.rb` and place it there, I can change it up.