API doc: fix end slashes and examples

Add end slashed to all urls.
Match examples with default-fixture.
Improve test strategy when creating users

Plus the tests did not work against PostgreSQL because of a mistake: the id of the newly created user was hard
coded as 1, instead of being read with user.id.
Merge pull request #243 from nimiq/api

Help page and ending slashed fixed
Merge branch 'api'
datadump update