add project avatar image to Project index and show pages

* added :avatar_url field to Project table for efficiency
* project avatar image replaces octocat on Project show page
* project avatar url updates in Project#update_repository_info
add cross-dev Gemfile

* added cross-dev Gemfile
* fixed some typos
* updated dev wiki link in README.md
* updated 'bootstrap_form' gem in Gemfile

the 'sigmike/bootstrap_form' gem added to the Gemfile in
      https://github.com/tip4commit/tip4commit/pull/74
  has been merged upstream
      https://github.com/bootstrap-ruby/rails-bootstrap-forms/pull/76
  it was added apparently as a bugfix but is now quite outdated
  it was upgraded in the peer4commit repo in june
      https://github.com/sigmike/peer4commit/commit/
              608980f7bd7b9fde5366180142f050d7baaf27a1
merge latest upstream master
Blacklist my personal (mpasternacki) and my organization's (3ofcoins) repos
Merge pull request #181 from mpasternacki/master

Blacklist my personal (mpasternacki) and my organization's (3ofcoins) repos
fixing #182; also added some text to warning to prevent further confusion
Merge pull request #183 from nalesnikld/master

fixed polish translation
Merge pull request #144 from bits4commits/dev-tweaks

add cross-dev Gemfile and update 'bootstrap_form' gem
updated Gemfile.lock
Update hr.yml
Merge pull request #184 from win32re/patch-4

Update hr.yml
Admins can set minimum tip amount in the configuration file
Blacklist several projects
Update pl.yml
Merge pull request #187 from SirCmpwn/master

Blacklist projects I have authority under
Merge pull request #188 from ireun/patch-2

Update pl.yml
Polished up Polish translation :)
Merge pull request #189 from ireun/patch-3

Polished up Polish translation :)
Merge pull request #140 from bits4commits/project-avatar

add project avatar image to Project index and show pages
fixed collobarators