Add basic object model documentation

[hsorby]

Start of work on the object model documentation. Possibly a better place to discuss issue #9 since we can make use of Github restructured text rendering.

[nickerso]

• typo: AbstradtLlbCellMLError
• maybe typo: libcellml:format (should be libcellml::format?)
• I prefer the camel case version of libCellML to be LibCellml :)

[agarny]

Agreed with the camel case version: I also prefer LibCellml.

[nickerso]

I think this is sufficient to get on with those initial use-cases. And can now be merged in.

[jonc125]

+1 from me. It might be nice to have the source that was used to generate the diagrams as well though?

[agarny]

I am mainly concerned with the diagram for the first use cases at this stage, so in that context I am mainly fine with the current diagram. Indeed, I am not sure how deserialize() works. Why does it return a string? Also, shouldn't it take a string as an input? Either a URI or the contents of a CellML file? Maybe I missed the point of that method?...

Otherwise, I agree with @jonc125 that it would be nice to have the source for the diagrams, if possible.