Added file extension feature and cleaned code.
Merge branch 'master' of https://github.com/rgksugan/AndroidFileBrowser

Conflicts:
	src/ua/com/vassiliev/androidfilebrowser/FileBrowserActivity.java

Changes by Sugan Krishnan (https://github.com/rgksugan):
   Added option to filter by file extension and few code clean ups.
   Pull request of 11-Jan-2013.
1. Cleaning of code.
2. DeLinting code.
1. Added indication of free space to the select button.
(not working on non SD card paths for some reason).
2. Dealt with "Select" button layout problem, when there are two lines of text (basically, just wrapped each button to own LinearLayout).
1. Image of "Up" button returned.
2. Dealt with issue of 0 free space in system directories, where application cannot write. Now a proper message is displayed on "Select" button.