Fix not expression

* Rebased https://github.com/sass/libsass/issues/368 onto current master
* added in_declaration property to context to avoid parsing not expression in declaration

This fixes the issue in https://github.com/sass/libsass/issues/368 mentioned by @akhleung. However only for the example he gave, not for others of the "different set of cases".

#### Open Quesions:
Is this the way how this should be implemented?
If not - how else to pass in context information?
What other cases have to be thought of?