Add support to use mapping file categories

User can now specify a category in a mapping file to create the
semivariograms and color them by that field.
Merge branch 'master' of git://github.com/qiime/qiime into semivariogram_updates
Add functionality to create a color-legend
Add get_qiime_hex_string_color

Retrieves a hexadecimal string for use with matplotlib, etc.
Add usage examples and sample test data

Change some strings to make them fit 80 columns.
Merge pull request #986 from ElDeveloper/semivariogram_updates

plot_semivariogram.py updates