customisable font sizes in networkplots

[cwehmeyer]

• the parameter size now overwrites the matplotlib font.size default (instead of a hard-coded value of 14pt) and sets the label sizes for states and arrows.
• the new parameter arrow_label_size is only used for the arrow labels' font size
• the function plot_network() now accepts **textkwargs (like plot_flux() and plot_markov_model())

[franknoe]

thanks

[marscher]

Thanks for tidying this up! Appveyor failed for a strange linker error...