Add rdiff-backup optional backup worlds

Add some settings in msm.conf :
- boolean flags to enable or not zip original backup system or
rdiff-backup
- path where to store rdiff-backup world backups
- number of days to keep
- nice parameter
Merge pull request #221 from boozaa/master

Add rdiff-backup optional backup worlds
Changed hardcoded user home directory from '/opt/msm' to '$msm_dir'

When installing with to a non-standard directory, errors are thrown due to user being created with a home directory that doesn't exist. Fix sets user home directory to $msm_dir instead of '/opt/msm'.
Merge pull request #224 from Akythius/patch-1

Changed hardcoded user home directory from '/opt/msm' to '$msm_dir'
Fixed path returned during `jargroup getlatest`.

When running `msm jargroup getlatest <name>`, msm was returning path of (removed) downloaded jar instead of existing jar when already up to date.
Install wrapper to automatically detect distribution

Detects Linux distribution, downloads and executes corresponding installation script.
Merge pull request #227 from Akythius/patch-2

Install wrapper to automatically detect distribution
Merge pull request #226 from Akythius/patch-1

Fixed path returned during `jargroup getlatest`.
Update redhat.sh
Added java as a package dependancy for installers/redhat.sh (needed to properly run jars)
Added option to make msm_user account a system account (resolving issue #194)
Merge branch 'master' of https://github.com/sk4ly/minecraft-server-manager