code/style improvements

Removed redundant command line arguments from server.py

Beginning to standardize on single-quoted strings unless necessary to
use double-quoted strings (easier to read and understand the code).

Beginning to standardize on tab-based indenting rather than space-based
indenting. Each tab is 3 spaces wide so that the tabs are not
ridiculously large.

Improved configurability, made documentation more easily
human-readable, minor variable name changes.

Rather than change the working directory from within server.py, pass
the argument into the TFTP or HTTP server (starting with TFTP, will
improve HTTP in later commit) so that the classes are more easily
callable by other modules.