Document toString usage

This commit implements the standard format for each UDT. The idea
behind having a standard set of toString format specifiers is to
make the library easier to use for the end user.

Each UDT will have a %s specifier to output a human-readable
format. Each will also have a %d format specifier to output
a diagnostic format for debugging purposes.

See also
http://wiki.dlang.org/Defining_custom_print_format_specifiers