Small thickness lines disappear at some orientation

Motivation behind the changes
---------------------------------------
By generating stimuli for illusory contour experiments, I have observed that, when the lines have a small thickness, they disappear at some orientation (particularly 0 and pi/2). See the test code below. The fact that it is orientation-dependent may be annoying. I propose in the pattern generator Line to check the thickness with respect to the pixel size. This change guarantees that, even with a thickness of 0, you will get a line
After this change, I propose to get a little bit further, that is to guarantee that the line of the smallest width that you get is always 1 pixel. Again, in the current code, you get this feature at nearly all orientations except close to 0 and pi/2. To get this feature at all orientations, I propose in this pull request a change in patternfn.py/line.

Test code
------------
import matplotlib.pyplot as plt
import matplotlib.cm as cm
from numpy import pi

import imagen
from dataviews.sheetviews.boundingregion import BoundingBox

bounds=BoundingBox(radius = 6)
xdensity = 20
ydensity = 20

fig = plt.figure()

def plot(image):
  im = plt.imshow(image,cmap=cm.Greys, interpolation='none', aspect='auto', extent=[-6,6,-6,6])
  
def one_line(orientation=pi/4):
  ima = imagen.Line(scale=-1,offset=1,orientation=orientation,thickness=0.03,x=0., y=0., smoothing=0.0, xdensity=xdensity, ydensity=ydensity,bounds=bounds)
  return ima()

plt.ylim([0,1])

for i in xrange(1,7):
  ax=plt.subplot(2,3,i)
  angle=(i-1)/10.
  ax.set_title('angle='+str(angle)+'pi')
  plot(one_line(orientation=(angle*pi)))
  
plt.show()

Results of the test code
------------------------------
Initial behavior: at orientation 0 and pi/2, the lines disappear:
![line_disappear_ko](https://cloud.githubusercontent.com/assets/5656774/3190693/ca6d39d6-ecd1-11e3-8bf2-4c04f0ee0c43.png)

Intermediate behavior after the first correction: the minimal width is 2 pixels at 0 and pi/2, whereas it is 1 pixel at other orientations:
![minimal_line_wider_than_1_pixel_ko](https://cloud.githubusercontent.com/assets/5656774/3190698/cf88afea-ecd1-11e3-81e6-97fdc652eb1e.png)

Final desired behavior: minimal line width is one pixel at all orientations (the sixth panel has a line of width one, not visible here due to the resolution):
![minimal_thickness__one_pixel_ok](https://cloud.githubusercontent.com/assets/5656774/3190632/fb1f1f64-ecd0-11e3-8b94-7e9204b04a08.png)

It's great to have tests, but please rename "test" to "tests" (to match our other projects like param and dataviews) and remove the README file (as running the tests will be documented at the top level).  Thanks.