rebaseline_server: add --editable and --reload flags

rebaseline_server: add --editable and --reload flags

This is a variation on the --browseonly flag we came up with in
https://codereview.chromium.org/24274003/#msg6 .

The long-lived "view only" server will probably be run like this:
server.py --export --reload 300

And when rebaselining you'll run your own server like this:
server.py --editable

(SkipBuildbotRuns)

R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/27218005

git-svn-id: http://skia.googlecode.com/svn/trunk@11788 2bbb7eff-a529-9590-31e7-b0007b416f81