Rename HTTPClient namespace to HTTP

This should make namespace a bit more flexible in scope and allow us to
move more functionality there later.

Move deprecated HTTPClient classes to deprecated.php

PSR2 adjustments for the PassHash class

I opted for ignoring the camel case funtion check not because the public hash_*
methods are widely used but because I find this style actually cleaner
in this

PSR2 adjustments for the PassHash class

I opted for ignoring the camel case funtion check not because the public hash_*
methods are widely used but because I find this style actually cleaner
in this case where the method name is auto-built from the hash type
name.

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adjusted the Input clases for PSR2

They are now in their own namespace.

deprecated JSON class

JSON is natively supported since years. This makes
2145bd4c56435f829a19aad843172f8ddffbc69b obsolete again.

some small fixes for PSR2

split changelog classes into their own namespace

The remaining functions in inc/changelog.php should be moved into a
utility class.

fallback classes for plugins inheriting from old settings classes

This will still throw a signature mismatch warning on PHP7 but at least
it is no longer fatal.

started a dedicated file for deprecated stuff

PSR-2 compatibility for the remote API libs