Add Ex 12, EC 3 of LRTHW.

Add Exercise 12, Extra Credit 3 of Learn Ruby the Hard Way. Add script
that prints to screen the directories on the module search path.
Add Ex 13 of LRTHW.

Add Exercise 13 of Learn Ruby the Hard Way, which introduces ARGV.
Confirm completion of Ex 13, EC 1 of LRTHW.

Record completion of Exercise 13, Extra Credit 1 of Learn Ruby the
Hard Way by copying the output of running the script with two
arguments.
Add Ex 13, EC 2 of LRTHW with fewer arguments.

Add Exercise 13, Extra Credit 2 script with fewer arguments than
Exercise 13 script.
Add Ex 13, EC 2 of LRTHW with more arguments.

Add Exercise 13, Extra Credit 2 of Learn Ruby the Hard Way script
with more arguments (theoretically, an arbitrary number). Uses
a for loop.
Add Ex 13, EC 3 of LRTHW.

Add Exercise 13, Extra Credit 3 of Learn Ruby the Hard Way, which
combines getting input from the command line (via ARGV) with
getting input from STDIN.
Add version of Ex 13 of LRTHW with notes.

Add version of Exercise 13 of Learn Ruby the Hard Way with comments
above selected lines to reinforce some new concepts.