[Verse] is not in scope for the ps module, so removed the link in the
documentation comments.

Added documentation comments for the recite::ps::task and
recite::ps::ctask macros.

Removes the custom errors in src/recite/erro.rs, and replaces them with
std::io::Errors throughout (recite(), incant_, macros).

Fixed a bug with the way forking to the background is handled, where
registering the signal handler in main for all processes would break
couplets (i.e. pipes). Instead, this sets up a new signal handler each
time a process is forked into the background. It uses a Vec<i32> to keep
track of all the background processes.

First off, moved the giant match statements out of recite(), and into
macros in src/recite/ps.rs. There still needs to be two, since any verse
using the 'couplet' meter will need to redirect its STDOUT. Now the
recite() function returns a Result<(), Mishap>, which can be invoked
when calling the incant_ functions.

Custom errors were added in the form of 'Mishap''s. They are intended to
be returned from the incant_ functions, in the event that something goes
wrong with the Command::spawn() or Child::wait(). They each take a
String, which should be the verb or stanza that was entered by the user.

The incant_ functions separate the functionality of each type of meter
from the recite() function. They return a Result<i32, Mishap>, where
i32 is the exit code of the program that ran, and Mishap is a possible
error.

Before, the shell was cheating at forking a process to the background.
It would actually spawn a thread to wait for that process to finish.
Now, the program simply registers a handler for SIGCHLD, and uses libc's
waitpid() function to reap the child process, and print some output to
the user, indicating that it's finished.

Broke out the structs for a poem into their own file: src/recite.rs.
Also put the 'prefresh' function into it's own file: src/recite/path.rs.
Commented most of the parser code (including structs and helper methods
related to parsing (i.e. Verse, Stanza, Meter, Poem)). Renamed any
instance of the 'paths' variable to 'path'.