Added util.heredoc(...)

- Included tests for util.inherit(...).from(...)
Moved .concat(...) method from StringValue to Value parent class as coercion applies to most value types
Added integration tests with initial implementation of PHP 'foreach' statement interpreter spec

- Includes additional methods for ArrayValue for management of internal array pointer
Wrapped code output in Mocha test name with JSON.stringify(...) to escape special characters, eg. newlines

- Prettifies test result output in console
Added PHP engine integration tests for 'foreach' statement
Wrapped string literal definitions in .unescapeString(...) tools call to handle string escape sequences