Implemented Nan and aligned to segfault_handler naming convention

This is a breaking change as it names the module segfault_handler with an underscore to make the build and sample code work. 
On the upside it implements Nan change and so should run on later version of node. 
Would recommend publishing in npm as segfault_handler too for consistency and maybe removing/deprecating the current 2.1 in there. 

It makes it consistent and I chose to go this way as this is how it's defined in  
https://github.com/ddopson/node-segfault-handler/blob/master/src/segfault-handler.cpp#L97 
It is recommended that the addon name is consistent with the name of the module. 
More details here 

http://nodejs.org/api/addons.html#addons_addon_patterns 
Specifically 
``` The module_name needs to match the filename of the final binary (minus the .node suffix).```