proj4 does not accept utf8 projstr

[christophfink]

In kartograph/proj/proj4.py the constructor function __init__ accepts a projstr which later is used to initialise proj4's pyproj.Proj()
This string typically is a unicode string, at least on my system, because json.loads() produces unicode output.
Thus, it is necessary to cast projstr to str().

Example error output:

$ kartograph -P testjson -o test.svg
  cli.py, in render_map()
  71: K.generate(cfg, args.output, preview=args.preview, format=format, stylesheet=css)  kartograph.py, in generate()
  46: _map = Map(opts, self.layerCache, format=format)  map.py, in __init__()
  48: me.proj = me._init_projection()  map.py, in _init_projection()
  106: return projC(**p_opts)  proj/proj4.py, in __init__()
  11: self.proj = pyproj.Proj(projstr)  /usr/lib64/python2.7/site-packages/pyproj/__init__.py, in __new__()
  328: projstring = _dict2string(projparams)  /usr/lib64/python2.7/site-packages/pyproj/__init__.py, in _dict2string()
  555: for key,value in projparams.items():
'unicode' object has no attribute 'items'

with test.json containing the following crs information:

    "proj": {
        "projstr": "+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs",
        "id": "proj4"
    }