Revert custom JSON.stringify to fix the app in Firefox

The fancy stuff I did in 2a7c41c doesn't work in Firefox :( it generates
"double-stringified" data like this:

```
"{\n    \"coordinates\": [\n        {\n            \"x\": 8,\n
\"y\": 4\n        }\n    ],\n    \"commitsPerDay\": 2\n}"
```
Merge pull request #29 from nylen/fix-firefox-json

Revert custom JSON.stringify to fix the app in Firefox