Refactor URL processing/generation into separate module

This is done to prepare for improved permalink generation
for URLs containing special characters, as proposed in
issue #782
do not force the permalink to be a dir if it ends on .html

Fixes #798.
test per-post permalinks

Add tests for setting a permalink on a post (in YAML frontmatter),
both the "old way" (generating an index.html) and with an .html ex-
tension.
Shuffle around rendering of extracted excerpt so it can be liquified.
Will now have to render the post, not just transform it.
Merge pull request #1302 from mojombo/excerpt

Add support for liquid in post excerpts
Update history to reflect merge of #1302
Don't try to be smarter than the computer. Pass Excerpt#to_liquid to its parent post.
Supporting more kramdown options.
Deleted unused code.
Added a configuration warning to our docs for the two kramdown options we don't support.
Add optional time zone when specifying page.date.
Merge pull request #1304 from DamianLettie/PostTimeZoneDocs

Add optional time zone when specifying page.date.
Merge pull request #1201 from zachgersh/all_kramdown

Supporting more kramdown options.
Update history to reflect merge of #1201.
Fix for kramdown coderay options. #1201.
merge bang to modify in place. Woops. #1201
Release 1.1.0
Add Jekyll v1.1.0 release notes post.
Update history files.