Add MIT license text

Every FOSS project should have one :smile: 

Besides, it appears that we need one to get a free SSL cert (see #107).

Legal stuff: _technically_ we'd have to ask all previous contributors if they are willing to license their contributions under the MIT license (or whatever license we end up with). But, uhm, can we just skip this? :wink: 

-------------------
**Update**: Contributors (Checked: consents to having their contributions licensed under the MIT license)
* [x] svenfuchs
* [x] bastilian
* [x] carpodaster
* [x] joneslee85
* [x] pxlpnk
* [x] aishty
* [x] tjmcewan
* [x] AkshataDM
* [x] benedikt
* [x] lislis
* [x] cypher
* [x] sindhus
* [x] carlad
* [x] sareg0
* [x] lucaspinto
* [x] plexus
* [x] beanieboi
* [x] AlexTi
* [x] robinboening
* [x] anikalindtner

Contributors whose commits only included deletions:
* tbuehlmann
* patriciagao