Support for extracting bidirectional text

This pull request adds an bidirectional mode to pdfminer. In bidi-mode pdfminer
- recognises lines even if the glyphs are not adjacent to each other
- recreates the logical order of the text using the inverse Unicode bidirectional algorithm. This creates a dependency on package icu_bidi.

The bidi mode is controlled by the new command line options -B and -D of pdf2txt.py.

I'm not sure, if this patch is already ready for merging. Especially the dependency on icu_bidi could be a good reason not to merge this patch.