Dereferemce operators in both iterators are now const; this fixes iterator...

Dereferemce operators in both iterators are now const; this fixes iterator compatibility with parts of Boost Iterator (i.e. filter_iterator). It is now possible to damage the state of const_iterator by directly mutating internal handle - working around that introduces additional performance penalties and is generally extremely unlikely to be useful - constant iterator objects are rarely used.

git-svn-id: http://pugixml.googlecode.com/svn/trunk@858 99668b35-9821-0410-8761-19e4c4f06640