API REVIEW Axis label property

[Paru Somashekar]

JIRA : http://javafx-jira.kenai.com/browse/RT-28151

As the JIRA indicates, it is a simple change to use StringProperty for 
the axis label instead of ObjectProperty<String>. This is a breaking API 
change - however the likelihood is minimal, as both 
ObjectProperty<String> and StringProperty implement a lot of the same 
interfaces.
  As the JIRA notes, this is not consistent with the rest of the Charts 
API, where StringProperty is used instead of ObjectProperty<String>, 
hence the need to fix it.

thanks,
Paru.