Updated installation instructions

Split the installation instructions into different sections. Added a section to
explain how to do installation just for the project using Bundler.

Also added instructions how to run jekyll if Bundler has been used to install
the gems.
Track Gemfile.lock.
Aktualisierung der Code for München Projektseiten
Updated OK Lab Leipzig event
Merge pull request #199 from CodeforLeipzig/update-events

Updated OK Lab Leipzig event
Update events.yml
Update index.html
Merge pull request #189 from johnjohndoe/feature/add-gemfile-lock

Track Gemfile.lock.
Merge pull request #88 from CodeforLeipzig/update-installation-instructions

Updated installation instructions
Merge pull request #195 from konstin/gh-pages

Aktualisierung der Code for München Projektseiten
hamburg zuwendungen
Merge pull request #200 from kannes/gh-pages

hamburg zuwendungen
Show build status for Travis-CI.
Merge pull request #202 from johnjohndoe/add-build-status-image

Show build status for Travis-CI.
Update Ruby version to v.2.1.2.
Describe how to use RVM.
Update Termin OK Lab Hamburg
Merge pull request #203 from philippgeisler/patch-2

Update Termin OK Lab Hamburg
Merge pull request #201 from johnjohndoe/update-ruby-version

Update Ruby version to v.2.1.2.
summit blogpost