Add Travis configuration for Continuous Integration.

This unfortunately relies on some nasty hackery.

Travis environments are based on Ubuntu 12.04 (Precise Pangolin). The
repositories for this distribution only include ledger 2.6, which is
too old for our purposes (doesn't have the "accounts" command).

Attempting to install from a PPA failed due to unmet dependencies.
Instead, our Travis configuration first adds Debian's testing, unstable,
and experimental repositories to the Travis environment's sources.list,
and then installs ledger from experimental (testing and unstable are
used to satisfy dependencies).

This will, needless to say, break at some point in the future. We'll
have to deal with that when we get there.

Closes #1