Add missing Attach options including Uid, Gid, Namespaces and Personality.
Introduce DefaultAttachOptions

fixes #12
make go vet happy
make godoc happy
Add TestCommandWithUidGid
Add TestCommandWithArch
Add TestCommandWithEnvToKeep
define the values instead of including sys/personality.h
make go vet happy (Initialisms)
use the Personality type
Merge pull request #19 from caglar10ur/v2

Add missing Attach options