Add get! function

[emstoudenmire]

Add the get! function and adds a test of both get and get!.

Matt, I think you said something about a conversion being needed, but I reasoned that the Dictionaries library will handle this internally? Or did you mean a conversion of the edge or vertex types?

[mtfishman]

Ideally we would rely on whatever conversion is done by Dictionaries.jl.

[emstoudenmire]

Yes, great. That's what I thought too.

[mtfishman]

Looks good, thanks!