Allow headers to be set per request

Version 1.2.4

Merge pull request #64

Add options for json_encode function

Merge pull request #59 from vgarvardt/master

Changed ResponseBuilder members visibility from private to protected, reduced cyclomatic complexity of handleExceptions

Minor fixes

Added withLocalExceptions()

Added new callback before client request

Change exception handling in client

Cosmetic fixes

Change minimum requirements to PHP 5.3.4