Make a single function to parse CybOX files, and report errors sanely

[gtback]

Perhaps this should be cybox.parse. Eventually, we might add options for explicitly checking the version, etc. but for now there don't need to be any options.

The alternative is to put this function on Observables, so cybox.core.Observables.parse, since it should return an object of type Observables.

Note that you can currently parse a file by using the parseString function on the corresponding binding file, but this is not documented clearly and contradicts our attitude that users should not need to be aware of the bindings unless they really know what they are doing.

[treyka]

+1