Overload resolution

[AGBrown]

Related to #1

Need to work on overload resolution. Example:

A:

get<R extends ExistingDoc>(docId: string, options?: Options, callback?: async.Callback<R>): void;

vs.
B:

get<R extends ExistingDoc>(docId: string, options: Options, callback?: async.Callback<R>): void;
get<R extends ExistingDoc>(docId: string, callback?: async.Callback<R>): void;

Using A then the following code shows err in intellisense/type-checking as any. Using B err is correctly (and more helpfully) identified as Error | {}

db.get(info.id, (err) => {
    expect(err.error).to.exist;
    done();
});

[AGBrown]

Largely done - still need to add some correctness tests to pouchdb-tests.ts for one or two api methods.

[fredgalvao]

This should not be a rule, I'd guess this is a bug in intellisense on whichever IDE that is. Seeing this issue I now understand the reason for so many overloads 😄.