Pull in Mustermann

[Mustermann](https://github.com/rkh/mustermann) is the new pattern matcher for Sinatra 2.0. It can also be used as a plugin for Sinatra 1.x, assuming you are running on Ruby 2.0 or later.

Using Mustermann will fix #131.
This is probably more of a Mustermann question, but is this regex anchored? Also, GitHub allows usernames and repository names that are only digits, which could potentially conflict?

``` Ruby
>> /\d+/ === "foo123"
=> true
>> /\A\d+\z/ === "foo123"
=> false
```