Add scalar, object ref name override and auto id incrementation support

Some changes in Object Reference implementation:

1. Scalars are supported:
```
$str = 'hello';
$int = 12;
%div[$str] // generates <div class="string" id="string_hello">
%div[$int] // generates <div class="integer" id="integer_12">
```

2. Object ids are auto-incremented if there is no id methods:
```
$std = new stdClass;
%div[$std] // generates <div class="std_class" id="std_class_1">
%div[$std] // generates <div class="sdt_class" id="std_class_2">
```

3. Object Reference id can be overwritten with hamlObjectRef() method (see #23):
```
class foo {
  public function hamlObjectRef() {
    return 'customId';
  }
}
```