Optimized the way the labels are displayed on the X-Axis

The labels on the X-Axis are now offset to be centered if necessary and
possible.

v1.2.2

Replaced direct call of "layoutSubviews" by "setNeedsLayout".
Got rid of the extension .md of the "LICENSE" file for CocoaPods.

v1.2.1 - Getting project ready for CocoaPods

- Renamed the folder "BEMSimpleLineGraph" to "Classes".
- Modified the documentation to reflect the change of name.

Merge branch 'master' of https://github.com/Boris-Em/BEMSimpleLineGraph