Reorganize api specs

**spec api documentation:**
- move generated json to doc/api
- set aptitome title automatically from Rails app name
- use single page
- filter usefulness headers
- add CURL output
- sort examples how they listed in the specs
- update rspec_api_documentation to 3.0.0

**specs:**
- specs should be stored in the specs/acceptance
- each spec should be marked with `type: :api`
- set correct request header
- simplify Sessions API spec
- use `parse_json` instead of JSON.parse in the `spec_helper`