Add tooltip

[vgeorge]

Contributes to #36. This is a work in progress.

Changes:

• Enabled tooltip on hover for all layers
• Defined a common getTooltip() function that generates a formatted table with the feature properties
• Modified the build watcher to observe .css files

Issues:

• Feature properties don't appear to be available in info.object. Should we deserialized them from the arrow object?
• A small black square appears beneath the tooltip

cc @kylebarron

[kylebarron]

Should be fixed with my latest commit:

[kylebarron]

My changes:

• Set pickingRadius to 10 pixels to make it easier to pick small objects
• Return only the content of the html div from getTooltip
• Call toJSON() on the Arrow object
• Delete the geometry attribute from the tooltip

[kylebarron]

Feel free to merge whenever!