gettext supports supplying multiple languages using the colon character
$ LANGUAGE=pt_BR:pt_PT:en_US foo bar
which are used as fallback: when a translation for the first one is not
available, the second language is used. If no language contains
a translation for the string `msgid` is returned.

This patch adds this support into gotext.

1/ config struct
 - 'language' was renamed to 'languages' and is a slice of strings
 - 'storage' was renamed to 'locales' and is a slice of Locale pointers

2/ loadStorage()
 - all loaded languages are iterated over

3/ GetLanguages()
 - new function returns the languages from the config
 - GetLanguage() uses the first element of it, keeping the compatibility

4/ SetLanguage(), Configure()
 - the language string is split at colon and iterated over

5/ Get*()
 - languages are iterated and the first translation for given string is
   returned

6/ IsTranslated*()
 - new optional parameter (langs) has been added

7/ Locale.GetActualLanguage()
 - it checks the filesystem and determines what the actual language code
   is: for 'cs_CZ', just 'cs' may be returned, depending on the actual
   name of the .mo/.po file.

8/ GetLocales/GetStorage, SetLocales/SetStorage
 - Following recent changes, created public functions to manipulate with
   global configuration's locales. The *Storage functions are renamed
   in later commit to reduce the amount of changes in one commit.

As the package matured, it makes sense to use unified names everywhere.
Since the config holds Locale objects, the variable is called 'locales'.

This patch finishes the work and updates the 'loadStorage' function
to be called 'localLocales' with argument 'rebuildCache' instead of
'force'.