fix issues with ambiguous Language headers

po documentation allows the Language header to contain an abbreviated
version of the Language name.

Therefor example files simple.fr_FR.po and fr_CA.po have been added.
Both have "Language: fr\n" header set, which makes the target ambiguous.
The convention is to silently fall back to fr_FR in this case (only if
the filename contains the complete string, fr_FR, in any way) or to
fall back to the filename itself, if the previous fallback mechanism
did not work.
grunt: run clean task if any file changed

make sure to test against a clean tmp-directory, otherwise, there might
be old files lying around and strange things might happen during test run.
grunt: use separate watcher for Gruntfile.js

watcher needs to reload if Gruntfile.js changed
install grunt-bump as devDependency

makes release-management more easy