SAX-core #2: factories

[David Brownell <david-b@pacbell.net>]

This seems to me most like a bugfix, but I'm sending it for
possible discussion in case anyone has issues.

Factories

- Most distributions don't bother defining a default parser to
  use when the appropriate system property isn't set, or access
  to it is disallowed (applets etc), though that's allowed by
  the spec.  Net result, the SAX bootstrap API is excessively
  fragile with respect to environment and configuration.
  
  This encourages initting by calling constructors directly
  (and using nonstandard parse APIs), or using other bootstrap
  APIs (like JAXP) which provide robust defaults.  Neither of
  those is healthy for SAX.

PROPOSAL:

    - Update javadoc to say clearly that when the system property
      can't be accessed, an environment-specific default may be used.
    
    - Encourage distributions to provide such a default; it doesn't
      change the API, only increases robustness.