Dynamic tickValues

hi Masayuki,

first off, let me tell you how great C3 is. thanks for your hard work.

this pull request lets a user attach a function which will return calculated tickValues.
the function is provided with the current domain, so the function can adjust the tickValues bases on the zoom level.

here is a plunkr which demonstrates a use case: http://jsfiddle.net/chriskalmar/wSdJX/
in this example I'm using time series and depending on the zoom level the tickValues change accordingly.