add Travis CI build status
cleanup re: paths vs base url.

- Do not parse base URLs (only parse when path added)
- Avoid path addition (requires a reparse anyway)
- Default paths to /
- Prepend missing / to paths
- merge paths (from commandline and config file) early on
- Skip trailing / in local file paths
merge all config munging in one place

Happens in SiteDiff::Config::initialize():
- merge all config files
- merge global parts of the config file into before/after
- override configuration from run options, if necessary.
- specification for each of before and after (including base
  url and sanitization rules are moved to a separate small class)