Initial Travis CI configuration.

This commit introduces a Travis CI configuration for Levigo, which
will setup a semi-deterministic test environment for Levigo itself,
including dependencies and does not rely upon brittle assumptions of
Travis outside of build essentials and curl.
Fix golint warnings where practical.

This commit applies a number of trivial fixes across Levigo to
remedy golint warnings.  One change not associated with lint but
rather overall convention is the inclusion of the package name
prefix/breadcrumb in the emitted error strings.