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>