lib: Remove `.netrc` magic

This patch removes all `~/.netrc`-related logic from duo-package
entirely.  Configuration file parsing should be handled upstream in
`duo(1)`, not in a library.

Removing this magic will make adding credentials for other providers
significantly easier (and more verbose) when we add support for such
behaviour.

Closes #50.
Merge pull request #53 from stephenmathieson/remove/netrc-magic

lib: Remove `.netrc` magic