Rename 'ulong' to 'unsigned int'

That more closely follows the standard.
Merge pull request #301 from certik/visitor2

Polishing of visitors
Merge pull request #303 from certik/ulong

Rename 'ulong' to 'unsigned int'
Implement coeff for Add
Implement coeff for Symbol
Add a test for coeff
Implement coeff for Pow
Rearrange