Rename metavariables and improve documentation.

[Esper89]

Renames the $cond and $dft metavariables to $condition and $default, for clarity. Also improves documentation, fixes the broken doc test, and fixes a couple grammar and markdown mistakes in README.

[Esper89]

This pull request also removes myself from the authors field in the crate's manifest. The field is optional, vague, not actually used in most circumstances, and can't be changed in published versions of the crate. I'd recommend not using the field at all.