Merge pull request #35 from Sashixi/doxy

Doxy
Update the credits with more information
Fix an incorrect documentation reference
Fix two bugs with Parsing expressions
Fix a crash when an error expression contained the string "id"
Add find function to GenericPersistentUnit
Add a convenience method to get PersistentUnits from GenericTree
Add query to get root node of a persistent unit
Improve slightly the implementation of GenericPersistentUnit
Add a SimpleTextFileStore constructor allowing loading from GenericTree
Fix incorrect assertion when parsing expressions
Add ForAll and Exists to the contracts library
Add Logging to OOInteraction
Fix many bugs in parsing by enhancing the overall mechanism
Allow operator tokens of more than 2 symbols, e.g. @@@@@
Make it easier to adjust the signature of compound expressions
Add a test to parse all possible combination of tokens up to a length
Fix an incorrect assertion when parsing expressions
Improve the expression parsing tests
Clean NodeWrapper up a bit