Cannot post or put target images to AUVSI Competition Server

[bbridges]

In core/net/auvsi-client:AUVSIClient, postTargetImage() and putTargetImage() cannot upload images due to not encoding the images correctly. The images need to be encoded from base 64 strings to the correct format.

The server target image code can be found here. The error likely comes from the try block at line 398.

The following functions also cannot be tested properly because of this:

• getTargetImage()
• deleteTargetImage()

The tests in tests/auvsi-client/test-auvsi-client.js should be able to pass if the two functions are corrected.