Escape according to Canonical XML 1.0.

See <http://www.w3.org/TR/2000/WD-xml-c14n-20000119.html#charescaping>.

First and foremost, this change allows adding linefeeds, tabs and
carriage returns to attribute values.  This was not possible before,
since such characters did not get escaped and without escaping a
conforming XML reader will replace them by blanks. [1]

While at it, this change escapes attribute values, text, and comments
according to the Canonical XML 1.0 specification.  In particular, in
attribute values greater-than and single quote no longer get escaped; in
element values, single and double quotes no longer get escaped but
carriage return does get escaped; and in comments, nothing gets escaped.

[1] http://www.w3.org/TR/2004/REC-xml-20040204/#AVNormalize
Merge pull request #54 from jscheid/master

Escape according to Canonical XML 1.0