refactor use attributes
begin tests
clear container before appending
move capability checking to utils; move svg to utils
prevent overwriting container contents in some cases, clear in others
clean up
add tests; add type validation; fix container contents check; return added symbol element
release 0.8.0
Merge branch 'master' into gh-pages

* master:
  release 0.8.0
  add tests; add type validation; fix container contents check; return added symbol element
  clean up
  prevent overwriting container contents in some cases, clear in others
  move capability checking to utils; move svg to utils
  clear container before appending
  begin tests
  refactor use attributes