Triples

Two small additions to `DependencyGraph`:

* `zero_based` option to `__init__` and `load`, to allow input with zero-based node numbering. For some reason, [zpar](http://www.sutd.edu.sg/cmsresource/faculty/yuezhang/zpar.html) produces output that is almost in Malt-TAB format except that the first word is number 0 rather than 1.

* `triples()` method returns all the dependency triples in a `DependencyGraph`, where a triple has the form ((*head-word*, *head-ctag*), *relation*, (*dependent-word*, *dependent-ctag*)).



consider using named tuples here.
* dependency
  * head
  * relation
  * dependent

head and dependent are also named tuples with the following fields
* token
* tag

though i'm not sure what the best names are, would be great to be consistent with malt documentation.