adding support for port ranges on --expose

Addresses #1834

Signed-off-by: Srini Brahmaroutu <srbrahma@us.ibm.com>
I agree with @cpuguy83 - `expose` indicates to docker that there is something that it can 'link' - I find it a bit confused because I can set up `docker run -p 1234:2134` on non-exposed ports too.