Constrain stylus @import processing to styl files

Ensure that only .styl files get processed for @import statements, so
that stylus can more peacefully co-exist with SASS in the same asset
environment.
Fix test for more restrictive @import processor

Now requires .stylus_file? to return true in order to execute the
depend_on codepath, so let's fake that.
Add a test for .styl-restricted processing

Now asserts that we never process @imports on non-.styl files.
Use `file`, simplify non-styl example

Use Tilt's built-in `file` method instead of context.pathname; this
allows us to use the filename of the thing under test in the relevant
example, so use that also.
Use fnmatch instead of Environment#attributes_for

Turns out that Sprockets::Environment#attributes_for is internal to
Sprockets, so let's use File.fnmatch to determine if a file contains a
.styl extension or not.
Merge pull request #60 from antifuchs/restrict-import-processor-to-styl

Restrict import processor to .styl files