Fixes #2 GCC 4.9.0 and NAN v2.x syntax issues

[MatthewRalston]

I encountered some issues installing bgzf on CentOS 6 using Node v4. Eventually I discovered that the issues were related to my C compiler and the library NAN. The addresses issue #2 .

1. I needed to specify more modern C++ libraries, such as 4.9.0 in my environment.
2. The * character in package.json for Nan was replaced with major release v2, and the syntax has changed dramatically.
3. I updated the function declarations/exports to match the Nan v2 syntax.

[shinout]

Thanks for using this repo. Looks good to me except that shrinkwrap.json is included. Is it really necessary?

[MatthewRalston]

Necessary? Well, the main reason this became an issue in the first place is because of the '*' in the Nan dependency. Shinkwrapping ensures that this dependency will run as is when downloaded in the future, guaranteed. Id you prefer that this is removed, then I will do so.