Expose RDFLoader class to global context + Pass file format information from Loader to Parser + fix typo

[ashleysommer]

This is a rollup of three changes I have made to rdfstore-js sources while working on my current project.

I needed to override a prototype function on the RDFLoader Class from within another JS library, to add additional functionality. In order to do that I needed to expose RDFLoader to the global context. Several other classes such as RDFModel and QueryEngine are exposed globally so I think RDFLoader should also be accessible.

The second change is to pass a format parameter in the options struct from the RDFLoader to the selected parser via the tryToParse method. The N3 parser needs this option passed in to determine whether the file is Turtle or N3 in order to set some internal parsing options.

The final change is a small but nasty typo I found, which affects the use of the store.load helper.

[bensinober]

I backup @ashleysommer's PR, it is a necessary fix. Please merge!