Change startup behavior to match that of 'jekyll serve'

[stomar]

Change startup behavior (of Rack::Jekyll.new):

• Adopt the current behavior of Jekyll (the "jekyll serve" command) and always build the site initially.
• Add an option to skip the initial build (e.g. :skip_initial_build - this would match Jekyll's command line flag --skip-initial-build).

The option is necessary for deploys on read-only filesystems, where the pre-built site already exists. (The current :force_build option, which is only available since 0.4.2, will be obsolete.)