Property for labels always visible & misc bug fix

- Added property `alwaysDisplayPopUpLabels`, for pop up labels to
always be visible (see issue #28).
- Changed `arrayOfPoints` from NSMutableArray to simple NSArray,
hopping that it will fix issue #29.
Merge branch 'feature' of https://github.com/Boris-Em/BEMSimpleLineGraph into feature

Conflicts:
	Classes/BEMSimpleLineGraphView.m