Tests for nested maps, map-remove, multi-assign...

Various tests. Sorry, it's my first pull request I get it now, I should have made a branch for each one.

1. test for a parsing error wrt nested map syntax reported on the libsass PR

http://github.com/sass/libsass/pull/481

2. test for ruby sass 3.4 compliant map-remove() functionality.

https://github.com/sass/libsass/issues/510

3. test for multi assignment with `@each` directive sass/libsass#394

4. re-enabled the basic test for trailing commas, since they work now 

https://github.com/sass/libsass/pull/385