Update _barh.py

Support for stacked bars with option for annotating each stack.
Update _bar.py

Support for stacked bar chart and annotation each stack.
Update _bar.py

Added legend support.
Update _barh.py

Bug fixes and legend support.
Update _bar.py

Minor formatting fixes.
Update _barh.py

Formatting fixes for stacked support.
Merge pull request #24 from salilb/master

Stack support for bar and horizontal bar plots