Don't allow resource titles which aren't strings

It was possible to create resources whose titles weren't strings, by
using a variable containing a hash, or the result of a function which
doesn't return a string. This can cause problems resolving relationships
when the stringified version of the title differs between master and
agent.

Now we will only accept primitives, and will stringify them. That is:
string, symbol, number, boolean. Arrays or nested arrays will still be
flattened and used to create multiple resources. Any other value (for
instance: a hash) will cause a parse error.