Add __str__ and __add__ to qiime.util.MetadataMap

This will facilitate printing and merging of MetadataMap objects.
Modify merge_mapping_files library code

Removed write_mapping_data (one can now call print or str() on
MetadataMap objects)

Changed merge_mapping_files to make use of the newly added __add__ function
of the MetadataMap class.
Replace tests for new merge_mapping_file code
Fix script to work with new library code
Merge branch 'master' of https://github.com/qiime/qiime into improve_metadata_map

Conflicts:
	qiime/merge_mapping_files.py
	scripts/merge_mapping_files.py
	tests/test_merge_mapping_files.py
Accidentally deleted this file!
Update script interface
Merge branch 'improve_metadata_map' of github.com:adamrp/qiime into improve_metadata_map
Address comments from @jrrideout

Should be good to go, unless there are additional comments
Remove unneeded function import
modified to use bipy.core.sequence.DNA
modified to use bipy.core.sequence.DNA
modified to use bipy.core.sequence.DNA
modified to use bipy.core.sequence.DNA
modified to use bipy.core.sequence.DNA
Merge branch 'master' of github.com:qiime/qiime into remove-cogent-DNA

Conflicts:
	qiime/pycogent_backports/blat.py
Modified check_id_map.py to use IUPAC characters from bipy, issue #1421
modified to use bipy.core.sequence.DNA
modified to use bipy.core.sequence.DNA
removed dep on and refs to cogent.DNA