C functions with casts now accepted by the parser

C functions with a cast, e.g.:

```C
rb_define_method(cKlass, "method_name", (METHOD)function_name, 0);
```

are now accepted by the C parser (RDoc::Parser::C). It's just a change in the regular expression. A test is included.

This is necessary in projects that use C++, as G++ doesn't recognize functions correctly without the `METHOD` cast.