Bitcoin marketcap is only around 12 billion dollar so if few billionaire try to manipulate the market they can easily substract/add few billions which can swing price to +/- 10% easily within few hour as well as there are several events which can easily cause panic dump/pump, one of the latest example is yesterday's hoax about china limiting bitcoin trade cause price to drop from $740 to $685 within few hour.
recent events once again confirm the fact that China sets price for Bitcoin. The rest of the people to influence the Bitcoin can not
If you are a very big Bitcoin player I would imagine you could.
Any cryptocurrencies may be manipulated so that the price could be increased.