added check against canDelete BetterButton_Delete

This behavior should better mimic how SilverStripe allows deletion.
Even though you received a warning previously, this removes the Delete
button altogether when deletion is not allowed.
Update gridfield_betterbuttons.css

Drop width on css .better-buttons-utils so they don't overlay too much of the breadcrumbs
`checkVersioned` uses built in `hasExtension` function
`checkVersioned` checks there's more than one stage available
switching order of conditional parameters so simplest option fails first
Merge pull request #58 from dhensby/pulls/version-fix

Versioned fix
Rework `checkVersioned` to use built in `hasExtension` function and check more than one stage is available
switching order of conditional parameters so simplest option fails first
Merge pull request #59 from dhensby/pulls/version-fix

Pulls/version fix
Merge pull request #57 from andrewhoule/master

Update gridfield_betterbuttons.css
Merge pull request #56 from tylerkidd/master

added check against canDelete BetterButton_Delete