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.
Fixed issue with bezier curve

Fixed issue where the bezier curve used to be deactivated if 3 points
with the same value were following each other.
See issue #73.
move number generation in separate method
add Segment control to select Bezier or straight curve.
fix position of segment for iPhone 4
cleanup
Merge pull request #76 from skywinder/sampleImprovments

Add segment to select Bezier or straight line on sample app
Fixed issue #73 and labels Y axis height

- The issue with bezier curves should now be totally fixed.
- The height of the labels on the Y-axis has been increased to make
sure the could display all the text.
Merge pull request #78 from Boris-Em/pr/76

Fixed issue #73 and labels Y axis height
Fix several UI bug when displaying data with negative numbers

- Fix the method calculating max value of the data
- Fix the Y Axis Label Offset when displaying nagative numbers
- Fix the Pop Up Report when displaying nagative numbers
Add enable X-axis flag