Fix for Issue #8

Added a Custom Content Pane where we can change the icon/title of the
selected tab.

Added ReferenceCustomTabComponent which packs the tab title and icon
into a component.
Addressed in [commit](https://github.com/chetankothari/swing-htabs/commit/6bfca2c2651df7e4666bfe81991286bd7912bb6a)