Add support of /etc/debian_chroot

Debian uses the file /etc/debian_chroot to give a name to a chroot.
The default /etc/bash.bashrc contains:

if [ -z "${debian_chroot:-}" ] && [ -r /etc/debian_chroot ]; then
    debian_chroot=$(cat /etc/debian_chroot)
fi

PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '
Merge remote branch 'upstream/master'
Remove extra new line in case of SVN directory

Without the patch my prompt is (on 3 lines):
[rousseau:~/Documents/sc/nfc/libnfc]

svn ‡

With the patch my prompt is (on 1 line only):
[rousseau:~/Documents/sc/nfc/libnfc] svn ‡

This is because expr(1) adds a new line at the end of the result.
Merge branch 'master' of https://github.com/LudovicRousseau/liquidprompt into develop
clean chroot patch: add chroot for every connexions, modern test, local variabls
add an example of sourcing theme and ps1 config
Feature: blue @ when connected with X11 support

If the current connection supports X (if we are in a X session or if we have
enabled X11 forwarding under ssh), the @ displayed in front of the hostname
is colored in blue (use LP_COLOR_X11 to change the color).
Better default host colorscheme

Add two color for the X11 support, green (support) and yellow (no support, not
set in red, because it is not a critical warning). The bold cyan of the hostname
was to close to the green of the colon and the @, change it to blue.
fix git in a directory containing spaces
Merge branch 'master' of https://github.com/flepied/liquidprompt into develop
move virtual env function in the path section, move the display from the front to the path section of the prompt, inside brackets, add flepied as an author, update the readme
fix issue #74 by adding an example bashrc
Merge branch 'master' of https://github.com/jorisv/liquidprompt into develop
Merge branch 'fix_git_basename' of https://github.com/dr4Ke/liquidprompt into develop

Conflicts:
	liquidprompt
don't process VCS function if root

they do not appear in the result anyway, and they can be quite slow
LP_ENABLE_VCS_ROOT to enable use of VCS features with root
LP_PS1_PREFIX used to add arbitrary prompt before anything else
added a CUSTOMIZING THE PROMPT section
Merge branch 'ps1-prefix' of https://github.com/dr4Ke/liquidprompt into develop
Default prefix put a classical prompt as title

Update the doc and themes accordingly