Added support for PropertyAttributes

This PR is based on #160 and adds overloaded functions for ```NanSetTemplate()```, ```NanSetPrototypeTemplate()``` and ```NanSetInstanceTemplate()``` to support 
```
enum  	PropertyAttribute { 
  None = 0, 
  ReadOnly = 1 << 0, 
  DontEnum = 1 << 1, 
  DontDelete = 1 << 2 
}
```
Test cases are included.