LogScale and customBars with negative values

When LogScale and customBars are enabled, it was verifying if the value was negative by using a <= on an array and then inserting a null. This caused exceptions in rollingAverage(), the value can't be null when using customBars.

(This a new patch based on the current code, since pull request #236 was getting old).
Add test for custom bars with negative values in logScale
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Update dygraph.js
Update dygraph.js
Merge pull request #250 from wimme/patch-4

LogScale and customBars with negative values