Add v2 version of the package

[paskal]

Please review commit by commit. The only change so far is making RefreshCache types explicit; before marking this release, we'll need to merge #193 into it and write a Readme upgrade note.

The changes for the upgrade would be small, but users would still need to change their code a little, and it's breaking the compatibility promise, hence the v2.

[coveralls]

Pull Request Test Coverage Report for Build 8552405558

Details

• 2561 of 3095 (82.75%) changed or added relevant lines in 24 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.746%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
v2/provider/service.go	37	41	90.24%
v2/provider/apple_pubkeys.go	91	97	93.81%
v2/middleware/auth.go	137	146	93.84%
v2/token/user.go	75	86	87.21%
v2/avatar/localfs.go	61	74	82.43%
v2/provider/direct.go	86	99	86.87%
v2/avatar/store.go	56	73	76.71%
v2/provider/verify.go	117	134	87.31%
v2/token/jwt.go	199	217	91.71%
v2/avatar/bolt.go	65	87	74.71%

Totals
Change from base Build 8393595426:	0.0%
Covered Lines:	2561
Relevant Lines:	3095

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💛 - Coveralls