Y-Axis labels

[Boris-Em]

Follow-up of the conversation on issue #30. See specifically point2.

The feature branch now supports Y-axis labels thanks to @japanconman (see commit b5f21a2 & PR #32).

This feature is still being tested but so far seems to be working pretty well.

@Diaonic pointed out on a conversation on Gitter that the labels show too many numbers after the decimals point.

[japanconman]

The issue is due to YAxis label text is solely based on [NSNumber stringValue] on the float-to-string conversion in drawYAxis method.

Possible enhancement will be pulling out the Y-Axis label content logic to datasource. This will open an option for user to define the Y-Axis label text.