Added IPv6 support for GeoIP location, while still retaining IPv4 functi... #316

IPv4 tracking depends on:

• GEOIP_ADDR
• GEOIP_CONTINENT_CODE
• GEOIP_COUNTRY_CODE
• GEOIP_COUNTRY_NAME

And IPv6 depends on:
GEOIP_ADDR
GEOIP_CONTINENT_CODE_V6
GEOIP_COUNTRY_CODE_V6
GEOIP_COUNTRY_NAME_V6

For the isAvailable() and isWorking() functions, added checks to see if the GEOIP_COUNTRY_CODE_V6 variable has been populated.

For getLocation, if each IPv4 variable is empty, check if the IPv6 equivalent is empty. If not, use the IPv6 equivalent instead.

Thanks for pull request! I pushed the code without typo in: https://github.com/piwik/piwik/commit/4e17f95f2edf06ddf532d5e212c5eeca11efd80b

I haven't tested the code yet, would you mind test that detection of IPv6 geo location work?