Extract summary logger from progress formatter (#59, #63)

Feature run summaries can now be reused among formatters.
Store URI on AST elements
Display failing scenario URIs in summary
Release 0.2.16
Display only master branch build status in README
Rename "summary logger" to "summarizer" (#59)
Extract common formatter methods (#59, #63)
Add pretty formatter (simplified, monochrome) (#59)
Release 0.2.17
Rename spec file
Extract developer documentation from README to CONTRIBUTE
Replace findit with walkdir (close #73)

This fixes step definition and support code loading on Windows.
Bump browserify
Update supported Node.js versions

* Remove 0.4
* Add 0.7
Add contributors
Release 0.2.18
Update contributor details
Refactor "summarizer" listener to summary formatter (#71)
Add "summary" formatter to available CLI formatters
Fix spec example description