handle errors in FF during initialization

these errors happen when the iframe showing svg-edit
is set to 'display:none'
use svgedit.browser.checkFeatures to re-check

this is useful in the case of Firefox with an invisible
iframe. In that case some feature tests will fail
until the iframe is visible.
whitespace clean up
Merge pull request #2 from concord-consortium/ff-error-handling

handle FF invisible iframe