Fixed multiple issues related to #62

- Added properties to control the color and alpha of the background of
the X and Y-axis.
- Fixed issue with float not displaying properly on Y-axis and on pop
up labels.
- Changed the line cap to Round to have a smoother line.
Added documentation and fixed CGFloat properties

- Added documentation for background axis
- Fixed CGFloat properties from assign to nonatomic
Update README.md
Fixed Issue with Color Background X-Axis and Frame

- See second part of issue #62
Update README.md
Fixed `enableYAxisLabel`

- Fixed the way the offset on the Y-Axis label is calculated when
`enableYAxisLabel` is set to NO.
- Fixed the way the labels are displayed when `enableYAxisLabel` is set
to NO.
- Deleted the calculation of the offset distance for Y-Axis Label
(since it was already done).
- Fixed the Y-Axis labels when `enableYAxisLabel` is set to NO.
- Fixed the position of the dots when `enableYAxisLabel` is set to NO.
Fixed 'numberOfYAxisLabelsOnLineGraph'

The method ‘numberOfYAxisLabelsOnLineGraph’ now works all the time and
not only when ‘autoScaleYAxis’ is set to NO.
Fixed alwaysDisplayPopUpLabels

- Fixed issue with `alwaysDisplayPopUpLabels` where the background
wouldn’t be displayed when the Y-axis was displayed (see issue #71).
- Changed name of property `labelYaxisOffset` to `YAxisLabelXOffset`.
- Fixed documentation of `autoScaleYAxis`.
Update README.md
Fixed popupLabel

- When the Y-Axis labels were activated, the popup label on the very
edge of the Y-Axis label would display over the axis.