better error message on date format failure
Merge pull request #81 from jbclements/better-date-error-message

better error message on date format failure
Mention filename in error message. Fixes #82.
Update license copyright date through 2014.
In README link to LICENSE. Closes #83.
Add Racket 6.0.1 to .travis.yml
Ignore foo.html when foo.scrbl exists in _src/posts. Fixes #87.

Rationale: If someone uses DrRacket to edit the scrbl file, DrRacket's
preview button will dump an html file in the same directory.

At the same time, we recently added support for html post files.

Before this commit, Frog would barf on the DrRacket-generated html file
because it lacked the title/date/tags "front matter".
Use `let/ec` to "flatten" read-post.

The nested `cond`s and `match`es were getting a bit out of hand. A few
judicious non-local "return"s make the logic clearer.
Use git to get latest install-racket.sh
Major redesign for minimal rebuild. Bump to v0.18

- Use a new .frog/build file to cache results of the previous build.

  It is relatively expensive to parse a markdown file and enhance its
  HTML with Pygments etc. Likewise evaluating a Scribble file. In some
  cases, the resulting HTML can often be reused as-is -- merely
  "re-flowed" through post-template.html and page-template.html.

  Also this can help us detect when a source file has been deleted, or
  a tag is no longer used by any post, and therefore the associated
  output files may be deleted.

- Use file timestamps to detect other dependencies.

As a result, the common case of adding a new blog post results in
simply needing to:

- generate the new page

- update any index pages and feeds for the tag(s) used by the new post

- adjust the previously-newest post -- since it's "newer post" link
  must change, by re-flowing its pre-parsed HTML through post-template
  and page-template.

Likewise, adding or removing a tag for a post simply results in
rebuilding that post, plus the index pages and feeds associated with
the tag.

A full build is no faster than before, but a full rebuild now rarely
happens.
Noisy doc cache msg only when < Racket 6.0.1, when it's slow.
No " (page 1)" suffix in title for indexes' _first_ page.
Mention .frog/build in README.md. cf commit c34a03f
Use hash-update!

More concise and more efficient than `(hash-set! ___ (hash-ref ___))`.
Trim unnecessary require.
Delete unused variable.
Flip conditional.

Should be N/A, but still.
Use Scribble @title for non-post pages. Closes #88.

If no @title, continue to use the first @section as the title, as
before.
Fix bug with commit 7730251
Add missing dependency for index pages and feeds.

The content of a post appears on index pages and feeds for the tags used
by the post.