Support bzip2 and lzma/xz compressed man pages

Add support for bzip2 and lzma/xz compressed man pages.  Support for bzip2 is
part of the Python standard library (at least for 2.7 and >=3.2), while lzma/xz
is only in Python >=3.3; however, there is a backports module for Python 2.7 and
3.2.