Fix Post#url escape

Post#url was escaped using CGI.escape.
When file name contains a space character, its url points to
non-existing URL.

For example, when we have a post named '2014-01-02-foo bar.md',
we expect its url to be '/2014/01/02/foo%20bar.html',
but it was actually '/2014/01/02/foo+bar.html'.

We now define Jekyll::URL.escape_path and Jekyll::URL.unescape_path,
and use them to escape and unescape Post#url

test/source/_posts/2014-03-22-escape-+ %20[].markdown

@@ -0,0 +1,6 @@
+---
+layout: default
+title: Plus space percent
+---
+
+Signs are nice