Readbits binarises only the sample points

The speed gain is just micro-optimisation, but the detection is increased
a lot, because OTSU finds a threshold to classify only the sampled pixels
rather than the whole tag image.

Readbits was also improved by computing the sampling positions directly from
the corners of the tag rather than an estimation of the corners of the data
matrix.
Lowered the limit on the size of found quads

Also fixed an exception happening in ReadBits when the corner of a tag is
ouside of the image (Refine can produce such corners)
More feedback on which tag fails the tests

Extracted the metadata about the test images (i.e. filename and which tags are
present on the image) into a separate file.
Added the list of tags that are known to be undetected, in order to easily get
a feedback on which ones are new false negatives, and which ones are new true
positives.