Encode/Decode access token

[jsonUK]

The access token contains extra data (including refresh token and expiry) that is required. Loading from file does not work without this data. Added json_encode/json_decode wrappers.

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[jsonUK]

I signed it!

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CLAs look good, thanks!

[jsonUK]

FYI, I realise why this didn't work for me, it was because the Google API Client in v2.0 changed how the access token was returned.

See Upgrading notes from Google-Api-Php-Client

Will probably still need updating for future users.