Strip out './' when looking up relative @imports.

This was causing JRuby to choke on templates in a jar importing each other.

Relevant Stack Overflow question: http://stackoverflow.com/questions/8236327/getting-file-not-found-error-running-sass-and-compass-as-jar-in-ant

Closes #583
Support comments within @imports in SCSS.

Closes #591
Fix an exponential parsing bug.

Fixes #604
Remove an incorrect statement from a warning.
Improve test_import_comments_in_imports.
Fix a broken test.
Bump VERSION to 3.2.4.
Don't generate bogus @extend warnings.

Closes #607
@import 'file' on windows fails with more than 1 subdir in the path

This commit changes lib/sass/importers/filesystem.rb and the method
find_real_file(dir, name, options) to ensure the 'dir' argument is
canonicalized to Ruby's standard "/" use for path separators.

Closes #594
Closes #619
try_send includes protected methods in respond_to?

respond_to? does not return true for protected methods in Ruby 2.0 unless you
explicitly pass true as a second argument.

Closes #572
Closes #611
Make Sass::Util.warn produce the same output on Ruby 2 that it does elsewhere.
Add a changelog entry about Ruby 2.0.0 compatibility.
Fix indented syntax line numbering with DOS lines.

Closes #588
Bump VERSION to 3.2.5.