Add newline to str; add Comments
Move test_str into MetadataMap tests in test_util
Move merge_mapping_files to MetadataMap object

It is now a staticmethod called mergeMappingFiles.  Also move test code into
test_util.py.  Remove qiime/merge_mapping_files.py and
tests/test_merge_mapping_files.py since the code has been moved into
qiime/util.py and tests/test_util.py, respectively.
Change import in scripts/merge_mapping_files.py

Now references code in the correct location (in qiime.util.Metadatamap)