Add "kpm add" command

parent #201 

Added command `kpm add [name] [version] [project]`. The `[project]` argument is optional and default value is current dir.
You can do `kpm add Newtonsoft.Json 5.0.2` to add `"Newtonsoft.Json": "5.0.2"` into `"dependencies"` section of project.json under current dir. If a `Newtonsoft.Json` dependency already exist in the `"dependencies"` section, the version information is overwritten.