Fix #84 Failing on unmatched quotes inside <?PI ?>

The XML spec defines processing instructions as:

PI ::= '<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'
PITarget ::= Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))

( http://www.w3.org/TR/REC-xml/#sec-pi )

This does not require quotes inside the processing instruction to match.
The code, however, was entering and exiting a special state
(PROC_INST_QUOTED) for quotes as it hit them. This caused it to
incorrectly error out on unmatched quotes.  This commit removes
PROC_INST_QUOTED.