21, a San Francisco-based startup dedicated to helping developers integrate Bitcoin into apps, announced that it is open sourcing its software.  21 enables developers to add bitcoin payments to any app with a single line of code.  21 ultimately aims to create a "machine payable web," or a platform where computers can use bitcoin to pay for APIs, thereby removing the friction of manual signups and payments.   Developers can now download 21 for free and use the software to earn bitcoin, integrate bitcoin into their apps, and connect to other machines on the network.