Major refactor of entire API client (InfinityFreeHosting#61)

* Big refactor of the entire API client

- Remove the separate Request classes, the Client just returns the Responses themselves.
- The Client functions now take their input parameters as function arguments, instead of using an array.
- Use custom exception classes for Guzzle errors and custom logic errors.
- Add isAvailable method to AvailabilityResponse, isSuccessful also returns true if a domain is not found but no error was returned.
- Add pre-commit with various linters.
- Completely rewrite the test suite to use Guzzle mocks.

* Remove Pint for PHP 7 support

* Downgrade php-cs-fixer for old PHP versions

* Enhance documentation

* Fix timeout settings

Removed MyOwnFreeHost API port to use trusted cert

Fix phpunit requirement for older PHP versions

Add return type to all setUp functions

Remove support for PHP 5.6 and 7.0

Add isFound function and improve parsing for GetDomainUser call

Update package name in README

improved phpdoc

Added check for json_decode result (InfinityFreeHosting#40)

* Added check for json_decode result

* Followed PSR-2 Coding Standart

* Added test

Fixup for PHP versions in TravisCI.