Fix P4.login() for `p4` >=2014.1
Fix P4.login() for `p4` >=2014.1

The p4 login command `p4 login -u <user>` is not respected by Perforce
client version 2014.1.  Fixing the order to `p4 -u <user> login`
reflects Perforce's docs better and *should* be backwards compatible,
but I do not have an older version to confirm.

Tested against Server version 2015.1.

I know it's just a fix, but given that it may break with older p4
versions, it might be prudent to roll the major release.

gcochard/p4-oo