Add native fs.copyFile and fs.copyFileSync to copy() and copySync()

[manidlou]

Ref: #490.

This is based against the develop branch.

Added native fs.copyFile and fs.copyFileSync to copy() and copySync().

[manidlou]

IDK what the problem is with travis! @jprichardson @RyanZim do you know what the issue is with travis?

[coveralls]

Coverage decreased (-0.6%) to 86.961% when pulling 0bd5279 on native-copyFile into 166f4a7 on develop.

[RyanZim]

Restarted the build and it worked!

[RyanZim]

LGTM; tested locally on Node 8 and ensured that the native method is indeed being used in environments that support it without issues.

[coveralls]

Coverage decreased (-1.7%) to 85.828% when pulling 1abc2a3 on native-copyFile into 166f4a7 on develop.

[manidlou]

@jprichardson ping?!

[jprichardson]

Looks good. Should be bundled in next major along with copy() changes.