Added qiime.simsam.create_replicated_mapping_file.

Also added unit tests.
Added docstring.
Added -m/--mapping_fp option to simsam.py.
Existing unit tests now pass.
Added mention to ChangeLog.

Fixes #784.
Added default to -m/--mapping_fp option help text.
Merge pull request #786 from jrrideout/issue-784

Added -m/--mapping_fp option to simsam.py