Add two isType methods, for Promise and Generator

[zzq0826]

var promise1 = new Promise(function(resolve, reject) {
setTimeout(resolve, 100, 'foo');
});
_.isPromise(promise1)
//[object Promise]

function* gen() {
yield 1;
yield 2;
yield 3;
}
let g = gen();
_.isGenerator(g)
//[object Generator]

[jgonggrijp]

@zzq0826 Thank you for your contribution and apologies for the long silence. You'd be welcome to finish this PR. The structure of upstream Underscore has changed considerably in the meanwhile, however, so you'd basically have to start over:

1. git checkout master.
2. Pull latest version of the master branch into your local clone.
3. npm run install: this updates the dev dependencies and installs commit hooks.
4. git checkout dev.
5. git reset --hard master.
6. Create modules/isGenerator.js and modules/isPromise.js. You can imitate the code from modules/isArrayBuffer.js.
7. Add isGenerator and isPromise to modules/index.js.
8. Add tests for isGenerator and isPromise to test/objects.js.
9. Add documentation for these functions to the index.html.

Please let me know whether you'd still like to make this contribution.