version 1.4
---------------
* [ADD] Added geom_edgelabel() to overlay labels on edges...
* [ADD] Added option to decorate_graph() to pass adjacency matrix and edges will be decorated with values 
  (for putting permutation probabilities on them)
* [ADD] Added permute_popgraph() to estiamte support for edges in the topology.
* [FIX] Changed write.popgraph() to write_popgraph() to conform with other naming conventions.
* [FIX] Added code to remove missing factor levels from popgraph()
* [ADD] Added option to decorate_graph() for default stratum="Stratum" as is common from data.frames 
  derived from stratum_coordinates() and other functions.
* [FIX] Fixed a problem when passing colors to geom_edgeset without a size characteristic.

version 1.3
---------------
* [FIX] fiddled with imports and importsFrom to get stuff to work without conflict between igraph and raster.
* [ADD] added minus operator for popgraph objects to find differences in edge sets.
* [FIX] changed all *population_graph() functions to *popgraph() for simplicity.
* [FIX] fixed popgraph() to ignore issues where log(1-SRI) -> NaN due to very small values of SRI


version 1.2
---------------
* [FIX] Changed geom_popgraph_edgeset() to geom_edgeset()
* [FIX] Changed geom_popgraph_nodeset() to geom_nodeset()
* [FIX] Changed decorate_popgraph() argument 'meta.stratum' to 'stratum'

version 1.1
---------------
* [ADD] Added function geom_popgraph_edgeset() to plot popgraph directly in a ggplot()
* [ADD] Added function geom_popgraph_nodeset() to plot popgraph directly in a ggplot()
* [ADD] Added theme_empty() for plotting graphs without background axes.




