Resolve "Get image metadata as dict"

Closes #49

The returned app.metadata returns something like:

{
    "AREA_OR_POINT": "Area",
    "DataType": 1.0,
    "DriverLongName": "GeoTIFF",
    "DriverShortName": "GTiff",
    "GeoTransform": [
        760056.0,
        6.0,
        0.0,
        6946092.0,
        0.0,
        -6.0
    ],
    "LowerLeftCorner": [
        760056.0,
        6944268.0
    ],
    "LowerRightCorner": [
        761562.0,
        6944268.0
    ],
    "ProjectionRef": "PROJCS[\"RGF93 v1 / Lambert-93\",\n...AUTHORITY[\"EPSG\",\"2154\"]]",
    "ResolutionFactor": 0,
    "SubDatasetIndex": 0,
    "TIFFTAG_SOFTWARE": "CSinG - 13 SEPTEMBRE 2012",
    "TileHintX": 251.0,
    "TileHintY": 8.0,
    "UpperLeftCorner": [
        760056.0,
        6946092.0
    ],
    "UpperRightCorner": [
        761562.0,
        6946092.0
    ]
}