Updated README and service.py for file type check

Updated README.md for the additional magic dependency

Updated service.py for the additional file type check

EDIT: Further optimized service.py on indentation issues and redundant structures. Updated comments for clarification.

EDIT 2: Further updated so that TXT files no longer go through PDF Box. PDF XML responses are now suppressed for TXT files.

EDIT 3: Updated bytestream handler to go through all current file type filters.

EDIT 4 (10/1/2013): Updated PostScript support using ps2txt, and some modifications on the XML responses.  

EDIT 5 (10/1/2013): Optimized code implementation and reorganized code structure. Readability is vastly improved now. Utility class and Extraction class are now separated from the service.py. 
 
--Lichi Li