Fix XML output to include spaces in "text" attribute

Sometimes whitespace is not explicitly defined in a PDF - it is just a gap between two characters. PDFMiner detects this and automatically inserts virtual spaces as "LTAnon" objects. Previously these weren't included in the "text" attribute of "textline" elements, which resulted in output that was missing all whitespace for some PDFs. This fix adds a single space for each "LTAnon" element, regardless of whether it contains multiple spaces or other whitespace characters. Leading and trailing spaces are also stripped off.
Fix a couple of bugs merged in from other branches

pdfinterp.py: python 3 stuff.
layout.py, utils.py: reverse a couple of optimizations from brechin that reduce the quality of output.
Allow overriding sys.argv to supply custom arguments to to command line utilities when calling main function from within python
Add unit tests for pdf2txt that run on PDFs in samples directory

The ".ref" files have all been updated so all tests should pass.
Switch custom setup.py build command to build_py so it runs even if setup.py install is run