Added `parse` method to malt.py, fixes #682

A quick fix to #682. Found an error in `.to_conll()` in dependencygraph.py too.

However, this does not make the Maltparser adhere to the API:

- `.parse()` returns a DependencyGraph, not an iterator of DependencyGraph
- similar for `.parse_sents()`, it returns a list of DependencyGraph, not an iterator of iterators
- `.parse_all()` and `.parse_one()` will raise an error

Stanford suffers from similar issues.