Implement namespace `Rakuten\WebService``

[ikari7789]

PEAR naming convention dropped in favor of namespacing. Fixes #13 and #14. Replaces #23.

Added

• Rakuten\WebService namespace.
• Development dependency on phpunit/phpunit=~7.5.

Changed

• Minimum supported PHP version 7.1.0.
• Mock object building logic in tests due to upgrade in PHPUnit version.

Removed

• PearHttpClient as PEAR library is discouraged in modern PHP development.

[hamadata]

Why travis CI is not working on this branch?