Sync cc.LabelBMFont to v3.0

Label Test presented some issues on BMFontMultiLine2Test:

The first letter after a line break was disappearing
Empty lines were not being shown
The functions used to calculate if a letter passed the max width were messed up
This pull request fixes them.

Commit by Bruno Assarisse
https://github.com/cocos2d/cocos2d-html5/pull/1714