Adds an API endpoint for Diff view

Provides HTML simple to understand diff output between two revisions 

IE:
http://10.0.0.219:9001/api/1.2.7/createDiff?apikey=EtherpadFTW&padID=test&startRev=1&endRev=5

Outputs:
```
{"code":0,"message":"ok","data":{"html":"<style>\n.authora_HKIv23mEbachFYfH {background-color: #a979d9}\n.authora_n4gEeMLsv1GivNeh {background-color: #a9b5d9}\n.removed {text-decoration: line-through; -ms-filter:'progid:DXImageTransform.Microsoft.Alpha(Opacity=80)'; filter: alpha(opacity=80); opacity: 0.8; }\n</style>Welcome to Etherpad Lite!<br><br>This pad text is synchronized as you type, so that everyone viewing this page sees the same text. This allows you to collaborate seamlessly on documents!<br><br>Get involved with Etherpad at <a href=\"http&#x3a;&#x2F;&#x2F;etherpad&#x2e;org\">http:&#x2F;&#x2F;etherpad.org</a><br><span class=\"authora_HKIv23mEbachFYfH\">aw</span><br><br>","authors":["a.HKIv23mEbachFYfH",""]}}
```