Making $hl7Globals truly global

[skeemer]

The basic idea is to make HL7::$hl7Globals static, adding HL7::set*() (similar to the with* methods) and also using it in Message.

1. This would make it easier to set the global options once at application boot rather than having to specify it every time we use HL7 or Message.
2. I don't think this would be a breaking change except in cases where people are doing something non-standard.

I'm willing to do a PR if this is something that might be accepted.