Add line stroke customization parameters to style.py

Added stroke_width, stroke_style and stroke_dasharray to style.py -- defaults are such that the original style remains the same, but now one can further customize the look of the lines in their graphs. I also added the appropriate unit tests.

Sample:
```
import pygal

config = pygal.Config()

config.dots_size = 0

custom_style = pygal.style.Style(
	stroke_width = 3,
	stroke_style = "miter",
	stroke_dasharray = '5,5'
)

line_chart = pygal.Line(config, style = custom_style)
line_chart.title = 'Browser usage evolution (in %)'
line_chart.x_labels = map(str, range(2002, 2013))
line_chart.add('Firefox', [None, None, 0, 16.6,   25,   31, 36.4, 45.5, 46.3, 42.8, 37.1])
line_chart.add('Chrome',  [None, None, None, None, None, None,    0,  3.9, 10.8, 23.8, 35.3])
line_chart.add('IE',      [85.8, 84.6, 84.7, 74.5,   66, 58.6, 54.7, 44.8, 36.2, 26.6, 20.1])
line_chart.add('Others',  [14.2, 15.4, 15.3,  8.9,    9, 10.4,  8.9,  5.8,  6.7,  6.8,  7.5])
line_chart.render_in_browser()
```