Don't treat XML declarations as Processing Instructions

[Holzhaus]

trafficstars

XML declarations are not processing instructions. This fixes #174.

According to XML 1.0 (5th Edition), Section 2.6, the PI target names "xml", "XML" and so on are reserved.

Production Rule 17 states:

PITarget ::= Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))

[CodyReichert]

Just hit this myself.

Anything I can do to help get this one merged? The test cases look correct, but let me know if I can help get it ready for release.