Initial support for OpenWrt

Recognize and handle OpenWrt, a highly extensible Linux distribution for
embedded devices. Currently supported:
- OS detection
- OS version detection
- hostname and domain detection
- memory information
- is_openwrt
- package management with opkg via Rex::Pkg::OpenWrt
Fix typo
fixed path lookup if rexfile was called from a different path
Turn "eval your Rexfile" into a debug message

There is no need for this message to be printed on every standard run and it adds noise, which can be particularly annoying for cronjobs.
changed info to debug
claim if a feature a rexfile want to load doesn't exists and stop the execution - #177
added tmp_dir command to set tmp directory per rexfile. - #166
tests
add cron tests

0 should be treated as a valid value for minute, hour and day_of_week
fields.
Fix #189

Only fall back to the default value of "*" for minute, hour and day_of_week
crontab fields if the passed value is undefined.
fixes ownership bug of files written with sudo
Merge remote-tracking branch 'ferki/cron-zero-values' into 0.42
updated build files
Merge branch '0.42' into development
Merge remote-tracking branch 'ferki/openwrt' into development
Add tests for ip command output
Add _parse_ip subroutine

Extracts the following information by parsing ip command output for a
single network interface:
- MAC addess
- IP address
- CIDR prefix
- broadcast address
Converts the netmask from CIDR to dotted decimal notation before return.
Note: currently works with IPv4 only.
Make get_network_devices try to use ip command if available
Make get_network_configuration try to use ip command if available
fixed an issue with append_if_no_such_line when searching for a string containing a double quote