Fix SetMetadataSource predecessors

Added GenerateLocale as SetMetadataSource predecessor, to avoid any
potential locale warnings. Also fixed a space indentation flaw and a
string which was being formatted with a non-existent key-argument.

This closes #75.
Add Travis CI

By now, it will only test the coding style rules defined for the
project, which are basically a few exceptions to the PEP8.
PEP8 fixes
Add information about coding style checking

Added a note about this to the CONTRIBUTING.md file. [skip ci]