Document Init(), Parse(), etc #82

When a parser, p, for some grammar is created, it has a number of public methods such as p.Parse(), p.Init() and others. I could not find these enumerated anywhere (other than grepping the generated .peg.go file).

I could not find documentation for those methods either. Perhaps I overlooked something, but currently I am just trying to guess at their expected behavior by studying the _test.go files.