Added basic support for PHP global constants as values

- No 'define(...)' support yet
Added initial support for PHP 'define(...)' function
Added support for case-insensitivity argument to PHP 'define(...)' function
Fixed issue with PHP return statement inside namespace
Added support for defining and accessing PHP constants in other namespaces
Fixed handling of undefined PHP constants when accessed using namespace path syntax

- Undefined namespaced constants raise a fatal error, whereas unnamespaced ones only raise E_NOTICE
Added support for PHP array cast operator "(array) <value>"
Added initial support for PHP throw operator
PHP 'throw' is a statement, not an operator
Added scenario to check that Exceptions stored in PHP variables may be thrown correctly
Fixed support for PHP foreach over element of array stored in property (using complex expressions as loop objects)
Added support for '--dump-ast' option to binary

- Dumps a JSON representation of the AST to stdout instead of executing it
Bumped version
Added 'Using on the command line' section to README, including '--dump-ast' option
Fixed issue with PHP error messages missing the trailing newline
Added basic support for PHP 'include ...' expressions
Updated Mocha test dependency
Fixed support for PHP logical Not operator ("! <value>") in grammar
Fixed support for PHP logical Not operator '!' in interpreter
Fixed issue with variables being used as array key/index in PHP interpreter