Add firebase\jwt namespace

[Dashron]

Puts the jwt class under the \Firebase\JWT namespace.

There is more discussion here: #52
I chose this namespace scheme to match another firebase php library: https://github.com/firebase/firebase-token-generator-php/tree/master/src

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[Dashron]

Waiting on approval from my employer before signing the CLA. Will update you when I have more info.

[AustP]

@Dashron - Any update on this?

[Dashron]

Waiting on the individual authorized to sign on behalf of the company (or however it was worded in the doc) to actually sign the CLA. Everything else has been set up.

[Dashron]

I've signed it on behalf of Vimeo.

[robertdimarco]

@Dashron Thank you for the contribution! I'll get a release cut with this and another upcoming change shortly.

[mannutech]

Unable to find JWT class. Help to resolve this