Adding two new test cases for exclude configuration use cases.

Here are two new shoulda test cases for `test_entry_filter.rb`:

- The first ensures that an excluded directory is based on the source directory, so a subdirectory elsewhere by the same name will not be excluded. This test passes. In 1.4.3, if `css` was in exclude list, then `css` and the `css` inside `vendor/` would both be excluded, though only the former was intended for exclusion.

- The second is to check if excluding a directory by name will exclude the files in that directory. This test does not pass, though it seems inconsistent with my experience using Jekyll 1.4.3 (`css` and all the files it contains should be excluded).