Only load unittest module in bootstrap if not already loaded

The unittest module bootstrap should not make a second
Kohana::modules call unless required, as this may have
side-effects. For preference the unittest module should
already be loaded - eg by koharness.

Also updated so that the module can be loaded from
any path, so that we don't need separate support for
MODPATH vs vendor/kohana/unittest installs.
Update README and add travis build badges