Adds --skip=image_resize to 'wp import' command.

Gives import user the option to skip image thumbnail generation during
image attachment import. Image resizing is extremely CPU intensive and
makes imports run much slower, especially during test runs that don't
necessarily require all image sizes. Image resizing can always be
deferred post import by running the 'wp media regenerate' command.
Re-factored patch to work with PHP < 5.4.
Removed thumbnail detection logic.
Added functional test for new --skip=image_resize option. Also added option to command doc output.
Fixed functional test (import requires --authors param to run correctly).
Added newline to EOF.
Merge pull request #784 from ctayloroomphinc/master

Adds --skip=image_resize to 'wp import' command.