Implement Excel 5 RC4 stream decryption

The decryption functions and objects implemented here are are
based on the source of Spreadsheet-ParseExcel:
http://search.cpan.org/~jmcnamara/Spreadsheet-ParseExcel/
Merge pull request #261 from trvrnrth/excel5CryptoRC4

Add support for reading protected (RC4 encrypted) .xls files