base: eval is evil

* Using ast.literal_value instead of eval to parse user provided values.
  Safer and more reliable (bool("False") -> True)

* Removing the checks that the new type corresponds with the known type
  for the value.

* Adding some tests to invenio.base.

* Adding invenio.base to the list of packages to collect test suites
  from.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>