RSS-Bridge currently has to guess the queried context from the data
provided by the user. This, however, can cause issues for bridges
that have multiple contexts with conflicting parameters (i.e. none).
This commit adds context hinting to queries via '&context=<context>'
which can be omitted in which case the context is determined as before.
Adds a new class 'ParameterValidator' to replace the functions from
'validator.php', separating private functions from 'validateData' to
class private functions in the process.
Instead of echoing error messages, adds messages to a private variable,
accessible via 'getInvalidParameters'.
BridgeAbstract now adds invalid parameter names to the error message.
