0.7.0 Release

- [x] Support for inherited instance variables
- [x] Block variables for `object` and `collection` methods
- [x] Deprecation warning for `current_object`
- [x] Update readme with instructions and examples for breaking `attribute` method change and new recommended syntax instead of `current_object`

* **Deprecation:** Deprecate usage of `current_object` method with block variables for `object` and `collection` methods
* **Breaking change:** Node#attribute block syntax no longer inherits context from current_object. See 941608e7 for more.
* **Breaking change:** Drop ruby 1.8 support