Use Less Memory and Fix Text Mode on Windows/UNIX  #303

The following are changed to fix #303:

  1. hashsum pulls 512 KB chunks of the file into memory. This ends up taking 1 MB with a secondary buffer allocated for windows. hashsum is now able to hash files larger than the computer's available memory.
  2. Text no longer transforms to UTF-8. This allows hashing to work on binary files without specifying text mode. On Windows, it converts a Windows newline '\r\n' to the standard newline '\n'.
  3. Set default modes: Windows uses binary by default, text otherwise.

Tested above claims on Windows 7/Ubuntu 14.04.