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.
Change an "if" to more appropriate "elif"

I overlooked an "if" that should have been an "elif". Oops.
Only try "lzma" module on ImportError
Skip lzma/xz manpages if lzma module not available

Skip man pages compressed with lzma/xz if the lzma module is not available; also
print a corresponding diagnostic message.
Only print an error when an lzma/xz manpage occurs

Only print an error when an lzma/xz compressed man page occurs. Also, use
add_diagnostic instead of print.
Use manpath instead of man --path in man.fish function (as in commit
c7941fc).
Add postgres psql completion

Options and descriptions are from psql --help output. Database and username
completion based on the bash completion
Add postgres psql completion

Options and descriptions are from psql --help output. Database and username
completion based on the bash completion
New documentation UI, including tutorial.
Update make_tarball.sh to work on Darwin
Replace nextd/prevd <pre>s with \code
Bump version to 2.1.0
Update OS X make_pkg script to also produce fish.app
Document appending `/` feature.
Document appending `/` feature.
Escape characters in function names. Fixes #1031.
Fix #213. You can now remove autoloaded functions.

Oddly enough, the code is here, but is internal function.
Fix to enable momentum scrolling on iOS in new documentation
Fix for buffer overflows identified by libgmalloc
Bump version to 2.1.0