add project avatar image to Project index and show pages

this PR is a consolidation of the commits from PR #126
based on the current master branch

* 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
* refactored ProjectsController#show action
* extracted #show action redirect before_filter into ProjectsController#redirect_to_pretty_url
* refactored bitcoin address update procedure to split long lines
      and abstract some magic numbers into config/initializers/constants.rb
* extracted bitcoin address update procedure into Project#update_bitcoin_address