Add check for native clipboard support and code to use it, fixes #171

[hallvors]

This pull request aims to make sure we use the native clipboard implementation rather than Flash in browsers with sufficient support for that.

If feature detection indicates native support is available, it will add a simple click event listener rather than a Flash overlay. The click listener will in turn add a copy listener, fire execCommand('copy') and remove the copy listener again.

It works OK on this test case in the browsers I've tested:
http://hallvord.com/temp/moz/zeroclipboard/test.html
but it could do with much wider testing - more browser/platform combos, Flash enabled/disabled.

@JamesMGreene - review?

[coveralls]

Coverage decreased (-0.4%) to 62.16% when pulling 83f88da on hallvors:minimalNativeClipboardImpl into d88febe on zeroclipboard:master.

[coveralls]

Coverage decreased (-0.3%) to 62.195% when pulling 26a3aa6 on hallvors:minimalNativeClipboardImpl into d88febe on zeroclipboard:master.

[hallvors]

@JamesMGreene I fixed the merge conflict here. Guess test coverage is still falling slightly, sorry I have not had time to figure out how to fix that.

[coveralls]

Coverage decreased (-0.3%) to 61.931% when pulling 13821cc on hallvors:minimalNativeClipboardImpl into 5beb963 on zeroclipboard:master.