PSR-7 support, CLI interface

• Add support for PSR-7 requests (e.g. ->send() and ->sendAsync()) (See #14)
• Add CLI interface to PHAR release file (See #8, docs)
• Move away from using \Exception to more appropriate, and package specific exceptions (See #13)
• Use a custom User-Agent (See #10)
• Fix issue with string query args (See #12)
• Show request body when using the verbose handler (See #11)