Flowpipe

* Nodes can be added to Graph on creation
* Graph is no longer a Node
* metadata on Nodes is UPDATED not REPLACED
* Rename function to node decorator to Node
* Nodes can be accessed via index on graph
* StatsReporter to receive evaluation stats

- Concept of engines removed
- Testsuite improved
- support for python 2.6
- Cleanup

Features:
- Api:
- Create Nodes with arbitrary Inputs and Outputs
- Node can execute arbitrary python code
- Nodes can be connected via their inputs and outputs to form a Graph
- Graphs behave like Nodes and can be used within other Graphs
- Graph Evaluation in arbitrary environments (Engines)
- Graph is JSON serializable and deserializable
- Comprehensive string representation for Graph and Nodes