Support animation of the 'display' attribute.

For now, we support SVG's list of display values:-

http://www.w3.org/TR/SVG/painting.html#DisplayProperty
inline | block | list-item |
run-in | compact | marker |
table | inline-table | table-row-group | table-header-group |
table-footer-group | table-row | table-column-group | table-column |
table-cell | table-caption | none

Note that CSS also defines values for display:-

http://dev.w3.org/csswg/css-box/#display
inline | block | inline-block | list-item | run-in | compact | table |
inline-table | table-row-group | table-header-group | table-footer-group
| table-row | table-column-group | table-column | table-cell |
table-caption | ruby | ruby-base | ruby-text | ruby-base-group |
ruby-text-group | container | flex | inline-flex | none

http://dev.w3.org/csswg/css-grid/#grid-containers
grid | inline-grid

http://dev.w3.org/csswg/css-flexbox/#flex-containers
flex | inline-flex

http://www.w3.org/TR/CSS2/visuren.html#display-prop
inline | block | list-item | inline-block | table | inline-table |
table-row-group | table-header-group | table-footer-group | table-row |
table-column-group | table-column | table-cell | table-caption | none
Merge pull request #69 from ewilligers/display

Support animation of the 'display' attribute.