Make dygraph-tickers more independent

This will allow for dygraph-tickers.js to more easily be included without the rest of the Dygraphs source.

Note that Dygraph will still be undefined. Adding the following line adds a lint warning:
```javascript
Dygraph = Dygraph || {};
```
```
[jshint] Error(s) in dygraph-tickers.js:
Read only. (dygraph-tickers.js:65:1)
> Dygraph = Dygraph || {};
```
Not sure what to do about this part, or if anything needs to be done.