Added twelve different tests for the parser (Poem::read).

Added a match statement in the main parser loop that pushes a new verse
into the poem if a newline is found. This might happen if parsing a poem
from a file.

[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.

Added documentation comments for the Meter::incant_ functions,
describing how each function operates.

Added an index to keep track of what position each char is at in the
loop. Added a more verbose error message, which prints out the column
that the parse error was detected, as well as the glyph that threw the
error. Added more comments to the '&', whitespace, and char match
statements. Changed parser behavior, so that a ';' glyph without a
stanza does not cause a parser error.

Moved the 'couplet' and 'metered' variables into functions impl'd for
Verse. This cuts down on the boilerplate in the parsing loop, and also
makes it so that their match statements are only ran when needed, rather
than being called at every iteration of the loop.

Introduce a switch to break from recite() if the forked process returns
a non-zero exit code. The one except to this, is when using semicolons,
then we do not care if the previous command failed.

Reaping is currently handled by a signal handler, so this code is no
longer necessary.

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.