New version of Graphs.jl package

[Dahua]

I have been working on a new design of the graphs package based on the Boost Graph Library.

As of now, the BGL branch has been merged into the master of Graphs.jl package. As the interface has been substantially redesigned, it probably breaks codes that rely on the old design.

The new version depends on Julia 0.2 (due to the use of immutable) and the DataStructures.jl package.

Detailed documentation is here: http://people.csail.mit.edu/dhlin/graph_jl/index.html

Please check it out here and let me know if you have any feedback.