Comments: Add initial support and tests

 * This commit makes it possible to create
   XML commment nodes in nodejs.
 * A new XmlComment class is introduced
   which exports a method text() to get and set
   the comment content.
 * This patch also includes tests to assert comment
   creation and the getting and setting of comment
   content through the text() function.
Merge pull request #229 from lanoxx/master

Comments: Add initial support and tests
Use NAN 1.1.2

Node 0.11+ uses a version of v8 which has radically changed APIs. NAN is
a project to support legacy and currency versions of the v8 apis.
lock down nodeunit devDep
0.10.0
Merge branch 'master' from git://github.com/polotek/libxmljs into polotek-master

Conflicts:
	src/xml_document.cc
Merge branch 'polotek-master'