Add getters and setters for base directory

[stevekinney]

As of 0.2.0, you can just change the .base property. It would be cool to put in a getter and setter method that would allow us to add additional hooks later for things (e.g. fire an event when the base directory was changed).

var fileBin = new FileBin('/some/directory')

console.log(fileBin.getBaseDirectory()); // --> /some/directory;

fileBin.setBaseDirectory('/new/base');

console.log(fileBin.getBaseDirectory()); // --> /new/base;

[matt-stj]

@MattRooney and I will take this.