Symbolizers via python accepting **kwargs in constructor

[springmeyer]

It would be nice, via python to be able to create new symbolizers in the same fashion as datasources.

So you could either do:

params = {}
params['type'] = text
params['file'] = 'symbol.png'
params['width'] = 10
params['height'] = 10
text_sym = Symbolizer(**params)

or

TextSymbolizer(file='symbol.png',width....)

[springmeyer]

essentially, implementing symbolizers as 'property maps'

[springmeyer]

for context see #190

[springmeyer]

pushing to 0.7.0 milestone