Merge pull request #33 from japanconman/feature

Add Y-Axis feature
Clean up sample project
Minor change of X-axis behavior

- The X-Axis will now be drawn even if the method
`numberOfGapsBetweenLabelsOnLineGraph` is not implemented. All of the
label will be drawn, just as if it was returning 0.
- Clean up on ViewController.m
Added Reference Lines and Frames

Optionally draw vertical reference lines on the graph for each x-axis
label. Additionally, optionally draw a line above the x-axis to
separate the graph and the x-axis.
Master branch catch up & fixes

- Catch up with Master branch (PR#39)
- New method `graphLabelsForXAxis` that returns all of the labels on
the XAxis
- The position of line for the frame is now dependent of the label size
- The padding for the graph is now dependent of the label size
- Fix animation duration
Minor clean-up

- Got rid of `labelXaxisOffset`because it was not being used any more
Option to always display some of the pop up labels

- New method `lineGraph:alwaysDisplayPopUpAtIndex` that gives the
option to always display some of the pop up labels. See issue #28.
Remove overlapping labels is now optional

- New property `removeOverlappingLabelsOnAxis` making the feature that
automatically removes overlapping labels optional. See issue #30, point
3.