Make shell client work in OS X:

- use gnu env for `env -0`,
- use filesystem socket.
Update shell powerline client OSX requirements in documentation
Merge branch 'osx-shell-client' into develop
Explicitly print exception from :try block

Reason: :try takes any output to the stderr as the exception, so traceback is 
never printed even though nothing is caught.
Balance error message text
Do not emit a warning about symlinks when it is not the problem
Do not remove one component from directory path
Normalize all paths before doing anything else
Move one if check into python code

Reason: this removes one useless indentation level that breaks “tabs for
indentation, spaces for alignment” rule.
Merge pull request #1025 from ZyX-I/fix-vim-troubleshooting

Fix some issues in Vim troubleshooting code
Do not make daemon output zero byte

This byte is useless and can be seen in zsh output (strange that it did not 
cause bugs so far, at least none I know about). It may have been needed if any 
client was supposed to hold connection with daemon for a few runs, but all 
current clients work in “request → output response → exit” fashion without 
caring about terminating newline and definitely without preserving connection to 
daemon accross runs.
Merge pull request #1026 from ZyX-I/no-daemon-zero-byte

Do not make daemon output zero byte
Do not replace ascii with UTF-8 in powerline-daemon and python client

Assuming ascii was expected to be used when LANG=C it did not work in any case:
in this case preferred encoding is “ANSI_X3.4-1968”. Same for
`sys.getfilesystemencoding()`.
A few style improvements in Python client
Use posix.environ in Python client if available

This way there will be no need in converting keys and values to bytes objects on
\*nix systems.
Use the same function for getting encoding in Python client and daemon
Make powerline-render respect user locale as well

Closes #1023
Add `use_non_breaking_spaces` theme option, use it in ascii theme
Replace .replace(' ', NBSP) call with self.escape call

Dividers need to be escaped as well.

Fixes #1027
Fall back to ASCII-only theme in case of non-unicode locale