Javascript: Retrieving base64 image data from a web URL of an image

I am using the Pixabay API to retrieve images from their website. It states specifically that hotlinking is not allowed, and images should be downloaded and stored on your server (link to documentation): http://ift.tt/1BvoF26

If I have got the image URL by using their API, how would I convert it into its base64 data equivalent so I can store this data in my database?