Added function to change tab title/icon color when selected.

Currently, only the interactive indicator can be modified in the tabs just like on the Google Play Store.
This modification allows selected tab title/icons to be of different color, making the interface more flexible and intuitive.
I added some functions in the library, and applied them on the sample.

# library
 - pstsTabSwith(boolean) :  When a tab is selected, the drawable selector icon or the text color of the Tab title can be modified
 - pstsActivateTextColor(int) : The Title color of selected Tab
 - pstsDeactivateTextColor(int) : The Title color of unselected Tabs

I believe these additions will diversify the usage of this project, paving the way for a better interface.

Thank you for sharing a great open source.

Comments and suggestions are welcome!