Fix addAwesomeIcon:beforeTitle: when title is nil

If button title is nil, calling addAwesomeIcon:beforeTitle: would make
button title appear as '<icon> (null)'
Respect font size set in xib

Respect font size set in xib
Re-adding setting default font

Default font should be set in calls to initWithFrame but not
initWithCoder so that font size set in .xib is respected.
Merge pull request #4 from FigBug/master

Simple fix to addAwesomeIcon:beforeTitle: when title is nil