Adding ability to add labels to ids (such as nodes).

[pankajgupta]

We sometimes want to be able to store associated information with nodes and edges. This PR adds this facility and also includes a basic file reader for int node labels as an illustration. The file reading capability is not meant to be general (i.e., for all label types) at this point. Will also add an example to cassovary-examples next.

CC @szymonm @plofgren

[pankajgupta]

oh, failing travis ci build -- looks like I need to add a dependency in build file. Will do

[pankajgupta]

@szymonm looks okay?

[szymonm]

LGTM