Create SourceFileParser class.

The parsing logic is now encapsulated inside this separate class,
keeping the SourceFile class simpler and more to the point.  The
SourceFileParser is a singleton, so we no more create new merger,
doc-parser, etc for every single file we parse.

Also moved error handling out of SourceFile class up to App.  This way
the catching of parse errors no more interferes with unit tests.