add 10 to yLabelWidth on set, rather than on use

Padding on the line chart's y-axis labels can get crowded because when checking the x-axis label rotation (line 1506) it checks for `this.yLabelWidth + 8` rather than `this.yLabelWidth + 10`.
Ex:
![screen shot 2014-10-08 at 11 38 08](https://cloud.githubusercontent.com/assets/2278079/4562069/cc7e411a-4f01-11e4-97d3-9979ef334120.png)

Changing this check to `yLabelWidth + 10` fixes this issue.  However, since the `yLabelWidth` variable is either used to only check against itself (in the `fit()` method, lines 1462 & 1468), or checking against `yLabelWidth + 10` (line 1487 * 2, line 1506), I moved the addition to the setting of `yLabelWidth`, rather than when checking the variable.

Result:
![screen shot 2014-10-08 at 11 52 47](https://cloud.githubusercontent.com/assets/2278079/4562265/37381944-4f03-11e4-9f43-9e397055a442.png)

The result is correctly padded y-axis labels.