Doc: page about require()
Docs: list of api of some modules
Some webpage properties should depends on the document type

We should inject javascript only into html page.
refactorization of properties that returns content/plainText
Fixes #39: support of webpage.customHeaders

Added also the support of webpage.settings.userAgent (Issue #43)
Missing changes for issue #39. oups
Adds new option: --error-log-file=file

It allows to indicate a file where all javascript errors are stored
Fixed warnings and errors during tests executions