new function clean()

- removes texts (JSON objects) that are missing any of the specified fields

- new tests, and renamed old asynchronous tests
check methods

- check if download URL is working
- check if JSON field is filled
- better test organization
check.js; minor tweaks
fix test require paths and npm test command
extended check.hasField

now can check if a field has a certain value
another check test
simplified clean.js
random.text can be made to return only texts that have specified fields set, or set to a particular value
0.2.0