Fix TokenStorageInterface::clearToken()

All the current implementations of TokenStorageInterface::clearToken() seem to clear all tokens, not just one single service token, despite the comments and the method name leading one to think that only one token is deleted.

This patch fixes that by renaming the current clearToken methods to clearAllTokens, and adding a new clearTokenmethod that only clears the specific service token.

Fixes #75.

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This replaces https://github.com/Lusitanian/PHPoAuthLib/pull/79
It will also hopefully pass the tests, since the previous pull request was created while Github was under a DDoS and Travis couldn't get to the repositories.