Added support for setting timezone

A setting named timezone can be added to installer-config.txt which
will set the timezone of the installed system. The value (default is
Etc/UTC) must be a valid timezone identifier (e.g. Europe/Ljubljana)
Updated documentation wrt timezone setting.
Implemented setting the locale and updated the corresponding documentation.

Applied a modified patch from goranche, thanks for that!
Added code for multiple locales

The locale setting is renamed to locales and can hold a space delimited
list of locales that are to be enabled on the installed system.
All listed (valid) locales will be generated during installation
Added the system_default_locale setting

If the system_default_locale value is set, the value is passed to
update-locale, which in turn does the system specific setup so the
locale is used as a system default
(writes LANG=<locale> to /etc/default/locale)
Removed -z tests

replaced “! -z” tests with “!=“
Fixed locale enabling code

Forgot the /rootfs part while greping for specific locales… added
Added a space after the locale while uncommenting, so only the
specified locale is enabled (and not nl_NL* when only nl_NL is
specified)
Merge pull request #93 from goranche/multiple-locales

Changed the locale option to locales and added system_default_locale