Merge tag '0.15.0' into develop

This release combines fixes for a few issues on github.

* [#75](#75) Vertex.name, Edge.one, and Edge.two is now @PackageScope. This only
affects code that is @CompileStatic for now due to [GROOVY-3010](https://issues.apache.org/jira/browse/GROOVY-3010).
* [#74](#74 and edges may now be deleted. A vertex cannot be deleted if
there are edges referencing it.
* [#73](#73) Added EdgeWeightPlugin. This plugin adds the Weight trait to each
edge. Traversal methods are ordered by edge weight.

There were also several other changes that were not an issue on github:

Updated gradle to 3.5. Refactored gradle script to use the plugins closure when possible. gradle-gitflow does not work
with the closure because it is not in the gradle repository. This is another reason to update the plugin. Spock was also
updated to 1.1.

Added edgeTraits and vertexTraits. These methods will ensure traits are applied to current and future edges and vertices.

Added tests to provide more code coverage in jacoco.

Added more usage details to README.md

README.md

@@ -125,10 +125,10 @@ defaults for each, find, inject, findAll, and collect.
 
 Depth first traversal supports edge classification where an edge is classified as:
 
-tree-edge - when the destination vertex is white
-back-edge - when the destination vertex is grey
-forward-edge - when the destination vertex is black
-cross-edge - when the destination vertex is black and in a different tree
+* tree-edge - when the destination vertex is white
+* back-edge - when the destination vertex is grey
+* forward-edge - when the destination vertex is black
+* cross-edge - when the destination vertex is black and in a different tree
 
 ## EdgeWeightPlugin
 
@@ -195,7 +195,7 @@ If there are any issues contact me moaxcp@gmail.com.
 
 # Releases
 
-## x.x.x
+## 0.15.0
 
 This release combines fixes for a few issues on github.
 
@@ -206,7 +206,7 @@ there are edges referencing it.
 * [#73](https://github.com/moaxcp/graph-dsl/issues/73) Added EdgeWeightPlugin. This plugin adds the Weight trait to each 
 edge. Traversal methods are ordered by edge weight.
 
-There were also several other changes that were not on github.
+There were also several other changes that were not an issue on github:
 
 Updated gradle to 3.5. Refactored gradle script to use the plugins closure when possible. gradle-gitflow does not work 
 with the closure because it is not in the gradle repository. This is another reason to update the plugin. Spock was also