Removed .gitignore

.gitignore seemed to be very specific to the previous author and
shouldn't have been commited to upstream repository.
Added https to Gemfile and removed stale comment.

HTTPS is the recommendation for gem sources now, HTTP has a deprecation
warning.

Also removed a stale comment, a copy paste leftover?
Added ignore clause to the Guardfile template.

Guard should watch the entire source tree, but the default behavior when
previewing a jekyll site is that the _site directory is generated, so a
default guardfile should ignore this directory.
Removed UTF encoding

No purpose to explictly declare UTF8 encoding in this file, just leave
it as is by default.
Rewrote the entire Guard::Jekyll class.

Major rewrite of the Guard::Jekyll class.

- Use options hash instead of class variables.

- removed create_site, creating site on start was keeping Jekyll from
  picking up changes in the configuration files.

- Removed run_on_additions, run_on_modifications, run_on_removals
  because run_on_changes is called in all these cases.

- renamed jekyll! to build, jekyll! was a really non-descriptive name,
  also site is created in-place here using the current options.
Rewrote README

Rewrote README and changed its extension to its now more common
shorthand.
Merge pull request #9 from caspervonb/refactoring

Refactoring