Added utility to convert strings to CSS dimensions.

Added utils.parse_css_dimension, which takes an input int / string and
returns a valid CSS dimension. Added unittests.
Exposed height and width attributes to heatmap.

Height and width can now be set, both as keyword arguments to heatmap
and as properties once the widget is constructed.

Added a 'gmaps_traitlets' modules for traitlets specific to this
package. This now contains a CSSDimension traitlet for traits
representing CSS dimensions.
Merge pull request #13 from pbugnion/heatmap_options

Exposed height and width options for heatmap.