assets: SVG badge for Retina displays

A blue badge generated with http://shields.io/. Keeping the PNG for existing badges in the wild.
badge: baseline adjustment to SVG

https://github.com/badges/shields/commit/03e7013bf5f8e8bd207f27c826123e46bef31686
Fix log formatting error caught by `go vet ./...`.

gddo-server/main.go:257: possible formatting directive in Print call.
Make one log.Printf call consistent with all others by not adding \n at
the end (it's a noop, because log package always adds a \n at the end
if it's missing anyway).
Fix getGitHubDir failing sometimes because it didn't respect GitHub API pagination.

Fix #174.

If the response contains a Link header, then fallback to requesting
"refs/heads/master" and "refs/tags/go1" by name.

Use Go style for type naming.
See https://github.com/golang/gddo/pull/178#discussion_r14546920.

Modify to getJSON to return (*http.Response, error).
Check for the link header in GitHub specific code.
Excellent suggestions from
https://github.com/golang/gddo/pull/178#issuecomment-48007781. Thanks
@garyburd!
Merge pull request #168 from nathany/svg_badge

assets: SVG badge for Retina displays
Merge pull request #178 from shurcooL/fix/github-pagination-issue

Fix getGitHubDir failing sometimes because it didn't respect GitHub API pagination.
Don't display "Package is imported by n packages." for non-existing packages.

Fixes #179.
Merge pull request #180 from shurcooL/fix-imported-by-count-for-non-packages

Don't display "Package is imported by n packages." for non-existing packages.
Hide large literal values.

Display long strings as:

    "" /* %d byte string literal not displayed */,

See code.google.com/p/go.tools/godoc/static for an example of a long
string literal.

Display the elements of a long composite literal as:

    /* %d elements not displayed */

See github.com/tav/golly/prime for an example of a long composite
literal.
Improve gddo-admin stats command.

- Print header for each section.
- Print truncated packages.
Increase size limit for document truncation.

- Increase size limit from 200000 to 400000.
- Apply the size limit to the compressed size.
Merge pull request #183 from garyburd/master

Increase package size limit, decrease size of some packages