Support Plots.jl plots

[PhilipVinc]

Adds Requires.jl.
Remove LightGraphs dependency. (#39)
Support plotting Plots.

Missing tests ATM.
Should add some docs to this, otherwise people will never find out about it.

using TensorBoardLogger, Logging, Plots

p=plot(rand(100), rand(100));
tbl =TBLogger()
with_logger(tbl) do
  @info "first_plot" randomdata=p 
end
log_image(tbl, "another_one!", plot(1:10, exp.(1:10)))

[PhilipVinc]

Ok I think I have an idea, I'll try the following:

• Create a PNG struct, wrapping the binary data, that allows me to easily access things like height width etc...
• This is the only thing that has a image_summary. Everything else will be converted to PNG in preprocess

Since there is a lot of logic here that is duplicated between the preprocess and log_images I think it would be nice to start thinking about making all log_stuff functions trigger the logger internally. This would get ride of the code repetitions and make it more clear.

[PhilipVinc]

I abused TBImage a bit everywhere. Internally we now pass around either PNGs or TBImage(s) that get converted to PNGs with a rather intricate logic (that works well).
I'm starting to think that those TBObjects aren't really overrides, but rather useful objects to use inside the program to guide the dispatch...

Right now the folder structure and the files are a bit messy. Once this is merged I'd also like to have a stab at it.
I think we could split all the log_xxx methods in some separate folder, called ExplicitInterface or something, to be loaded at the end...

[PhilipVinc]

Ah, @xukai92 , I added native support for PyPlot too.

[xukai92]

This looks awesome!

[PhilipVinc]

I'd try to merge this soon, I think it's about time we tag a new release...

[PhilipVinc]

closes #39
Also adds PyPlot dependency.