Replace our HTTP client abstraction by Guzzle

[zimbatm]

It's not worth hitting the same issues when Guzzle already did all the work.

Bonus point for creating a Pusher mock around Guzzle's

Fixes #87

[GrahamCampbell]

👍

[rvanlaak]

Note: the Guzzle client to be used should be passable via the constructor as optional argument, so everyone can keep using their Guzzle middleware.

[WillSewell]

Could someone provide me with a list of benefits to end users of the library that using guzzle will bring?

[stale]

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[elverkilde]

This was released in version 6.