remove 'liblibxmljs' from bindings.gyp

The double lib causes problems for windows (and apprently was a hack
anyway). The compiled bindings can just be called xmljs.
add NODE_MDOULE macro invocation

This is the proper way node c++ modules should initialize themselves.
convert LXJS_GET_PARSER_FROM_CONTEXT into a function

Macros are inconsistent across platforms and bad practice unless no
other way is available. The uses of this macro are fine with a function
call.