(rather than each potential module having its own if check)

Set the stage for making the builtin modules more dynamic.

Note: this only converts crypto and net, I will add more extensions in a
later commit.

* node.h: Add utility macro for converting macro values to strings.

* node.h: Include the actual module name inside the module structure, not
  just the file it was built from.

* node.h: New Macro, NODE_MODULE_DECL, for declaring an external reference
  to a module structure.

* node_extensions.cc: New File, implements get_builtin_module, which
  iterates over the module structures that are compiled into node.

* node.cc(node::Binding): Use the new module lookup function to find
  modules.

* node_{net,crypto}.c: Add NODE_MODULEs to generate the module structure.

and added a test to expose it

Some compile time variables like the cflags and prefix have been moved to
the node_config.h.in, in the anticipation that they will be removed at
somepoint.

Added default values for many parameters, missing properties and methods,
and slight reorganization of the Child Process section.

Those variables have following defaults :
 - NODE_PREFIX is relative to the current path of the node_addon.py file
   It is used as the base path of node include files.
 - NODE_PATH is ~/.node_libraries
   It's where modules are installed when calling `node-waf install`
   Note .js files must be explicitely installed by the module's wscript.

Usage :
NODE_PREFIX=/usr/local NODE_PATH=~/.node_libraries node-waf configure

Added consistency to the representation of optional parameters and separated
out multiple functions that were in a single heading.

- Add docs for 'fd' events, Server.listenFD(), Stream.write(...[, fd])
  and http.Client 'upgrade' event.