Analyzing page fails when line has negative co-ordinates.

Some pdfs appear to have lines with negative x values, which raises
assertion error in Plane.drange.  This commit fixes the problem by
ensuring that we ignore such lines.

The current code fails with [this pdf](https://docs.google.com/file/d/0BxZ32wNgTCmtUUlITmYxNmZJZjg/edit?usp=drive_web&pli=1) but should pass with this fix. 

/cc: @baali