Tidied up the pepa_ast so that the names of the asttype are within the pepa_ast module. This means one cannot incorrectly create a ast node with the incorrect string for the asttype.
Further cleaned up the pepa_ast such that each *class* definition stores the asttype such that it can be used when one wishes to test the asttype of a particular node. For example one can do 'if node.asttype == PrefixNode.asttype' to determine whether a given node is a PrefixNode
Merge branch 'dev' of https://github.com/tdi/pyPEPA into dev
Merge pull request #14 from allanderek/dev

Small tidy of pepa_ast.py