Closes GH-56.

Creates new remotes if needed and fetches objects before cherry-picking.

    $ git cherry-pick http://github.com/mislav/REPO/commit/SHA
    > git remote add -f mislav git://github.com/mislav/REPO.git
    > git cherry-pick SHA

A remote must be an existing repo on GitHub.

    $ git fetch mislav
    > git remote add mislav git://github.com/mislav/REPO.git
    > git fetch mislav

If users keep the gem installed, they have a chance to be notified of updates
and they can refresh their standalone script. Also, executables in ~/bin should
have precedence over gem executables, so no need to uninstall.

New abilities:

  $ hub compare
  -> When on a branch that tracks "remotebranch", for example,
     this opens "github.com/user/repo/compare/remotebranch".
     In effect if the tracked branch is not "master".

  $ hub browse reque issues
  -> open http://github.com/user/resque/issues

  $ hub browse
  -> When on a branch that tracks "remotebranch", for example,
     this opens "github.com/user/repo/tree/remotebranch".
     In effect if the tracked branch is not "master".

  $ hub browse -- commits
  -> open http://github.com/user/repo/commits/master

  If this branch is tracking a remote branch:
  -> open http://github.com/user/repo/commits/remotebranch

Supported subpages:

  - "tree" (default)
  - "wiki"
  - "commits"
  - "issues", "network", "downloads" or any other arbitrary
    string is simply appended to URL path

Less juggling of constants, more ruby methods. Information that requires
shelling out (e.g. GitHub username, token, repo name ...) is now loaded
on demand, meaning no shelling out for commands that don't need it.

Tests are now safer; they are prevented from shelling out to get info.
All test data is stubbed.

New `github_url` method builds needed URLs while keeping sensible defaults.

Commands that open the browser (`compare`, `browse`) now share a bit of
underlying implementation and recognize the same flags (`-p`, `-u`).

This command allows you to view a compare view in the browser
from a START to an END commit.

Example:
  hub compare AWESOME_PATCH...MASTER

Usage example:

  $ git fork
  (forks "defunkt/hub" on GitHub)
  (git remote add mislav git@github.com:mislav/hub.git)
  new remote: mislav

Use `git fork --no-remote` to skip adding the new remote.

Tests use Webmock for stubbing out HTTP requests:

  $ gem install webmock

useful for jumping to the GitHub page of the repo checked out in the current directory

pages.

caveat: had to override jewelers install task