I am not sure but I think, technically its controlled by the community. What is the community? Consists of developers,miners, traders, those who hold currency.
1. Developers: You can find the Bitcoin source code here - bitcoin/bitcoin - it says there are 376 contributors, which you can see here - contributors - and you can make your own contributions / adjustments (they may or may not get approved)
2. Bitcoin miners: These people have bought the hardware / pay for the electricity to make the transactions and the network function.
3. Bitcoin traders: Large stake holders who move the markets in certain ways by altering liquidity.
4. Bitcoin holders: Those who are not necessarily in the trading realm, but have been buying and holding the currency.