Travis-CI: test also the zsh engine
README: fix typos
mksh: fix shell_code_static using corpus/special.PS1

spacial.PS1 is now 100% OK.
zsh: fix shell_code_static using corpus/special.PS1

In fact shell_code_static for zsh and mksh are now exactly the same
except for the ps1_finalize call for mksh.
POSIX/dash: do not escape any chars

As the client shell code directly outputs to STDOUT what the angel
produced, we don't need to escape anything.
Now corpus/special.PS1 passes.
ksh: only escape '!' to '!!' in the final result
corpus/special.PS1: also check tabs
mksh, zsh: fix shell_code_static for tabs
zsh: add compatibility with the standard zsh prompt engine

Zsh has already a prompt engine (see 'promptinit'). We now recognize it,
backup it, and disable it. When the angel leaves, we restore the prompt
theme.
bin/: fix shell name detection under cygwin
Shell: minor refactoring
AngelPS1::Sytem: "cygwin" is now an alias for "linux"
VCS plugin: fix cygwin compatibility in _find_vcs_dir

Use feature detection to disable a check on hard links count on
directories. This is an issue on cygwin because (stat $dir)[3] is
always 1.
VCS plugin: fix call to GitInfo

" which('git') " is now called just once instead of at every display
of the prompt in a Git directory.
Git plugin: use $shell_state->{GIT_DIR} if available

If someone (such as the VCS plugin) has already found the root of this
Git repo, use it!
add AngelPS1::Plugin::Term::screen (GNU screen support)
Add AngelPS1::Plugin::Term::tmux (tmux support)

... with an interactive test in corpus/tmux.PS1
::Plugin::Term::screen: POD fixes
README/build status: link to Github browser for each branch
Update copyright year