Feature/json meta image

[NTBlok]

Hi, I am new to making pull requests, but thought I'd give it a try. The reason that I am sending this PR is because I wanted to use the Glue repo to make sprite sheets for use with the pixi.js loader that expects a json format from TexturePacker. I found that if I set the GLUE_JSON_FORMAT='hash', I still need a meta key named "image" for the filename value. So, I simply added this key to the meta dictionary in the jsonformat.py file in the formats.

[coveralls]

Coverage remained the same at 92.494% when pulling 67fe44f427b17a09296c233f621936123e84e483 on NTBlok:feature/json-meta-image into 5c9cf65e9db80c47d3947255dfcf3b314cfe8c25 on jorgebastida:master.