Comcast announced today it is launching Stream, its own internet TV service. The $15 per month service will be available to Comcast’s Xfinity broadband customers and will first launch in Boston at the end of the summer before being made available everywhere in Comcast’s footprint by early 2016. By offering it in areas where only Xfinity is available, Stream will not, initially at least, be available to non-Comcast users. The service will offer a dozen networks via laptops and mobile devices, including the major broadcast networks and HBO. It will also offer on-demand content and a cloud-based DVR.