Merge pull request #81 from lxyu/cleanups

Cleanups
Merge pull request #82 from lxyu/string_padding

String padding
Merge pull request #83 from lxyu/ax_layer

add axis layer support
Merge pull request #85 from lxyu/pie_chart

Pie chart
Add pie/donut charts to examples, ipynb, readme, and docs
Fix Vega Bar Tutorial:
Update to 0.4