support user-specified ebcs, epbcs in Problem.save_ebc(), update docstring
normalize paths in top_dir, data_dir and base_dir
update import_file() to use full package paths for sfepy modules

- prevents name clashes for different fields.py files
add can_reload argument to import_file()
do not force module reload in load_classes()
fix issue #263, ConstantFunctionByRegion
Merge pull request #268 from vlukes/i263
fix Newton.__init__(), Oseen.__init__() to obey log plot option
WIP