[2.0] Implementing Content Graph

[carbontwelve]

This issue has grown out of #167 and #297 and effectively replaces them both.

While working on #297 I had begun rewriting Tapestry to use a tree data structure in order to store the content relationships. It very quickly became clear that a tree was the wrong data structure for the job and instead a more simpler graph data structure was substituted.

The benefit of using a graph data structure is that it can be visualised using GraphViz for debugging or outright geeky purposes.

This task has the following @todos:

• Write Graph data structure and accompanying unit tests
• Deprecate usages of Tree data structure and replace with Graph
• LexicalAnalysis needs to identify phtml layouts for the Graph, this needs a test (LexicalAnalysis needs to identify phtml layouts for the Graph #314)
• Have content generators add their dependencies to the graph (do these get added as a SimpleNode belonging to the configuration node? (Have content generators add their dependencies to the graph. #315)
• Have the graph be cached at the end of execution (Cache Content Graph at end of execution #316)
• Have the cached graph be loaded and at beginning of execution (Have the cached graph be loaded and at beginning of execution #317)
• Have the cached graph be compared with the generated graph so that it may be reduced and only changed files get re-compiled (Use cached graph to reduce the generated graph #318)