Allow Turntable to be initialized with default config

[RLovelett]

Most of the other Turntable initializers provide this convenience. Added it to this initializer as well.

(Discovered while working on #61 and #62)

If you look at the example in the comments of #61:

func testMocked() {
    let expectation = self.expectationWithDescription(#function)
    defer { self.waitForExpectationsWithTimeout(50.0, handler: nil) }

    let url = NSURL(string: "http://dfsdfhttpbin.org/hidden-basic-auth/:user/:passwd")!
    let error = NSError(domain: NSURLErrorDomain, code: NSURLErrorDNSLookupFailed, userInfo: nil)
    let track = [TrackFactory.createBadTrack(url, statusCode: 400, error: error)]
    let vinyl = Vinyl(tracks: track)
    let turntable = Turntable(vinyl: vinyl)

    turntable.dataTaskWithURL(url) { (data, response, error) in
        XCTAssertNil(data)
        XCTAssertNil(response)
        XCTAssertNotNil(error)
        print(error)
        expectation.fulfill()
    }.resume()
}

That let turntable = Turntable(vinyl: vinyl) is only possible with this change.

[RuiAAPeres]

👍