C4.5 algorithm added

- C4.5 algorithm added
- unittest added for C4.5

See [this ppt file](http://tesis-algoritmo-c45.googlecode.com/files/C45.ppt) for exact algorithm implemented.

brief for C4.5 is TBD
Handling missing data is TBD
Tree pruning is TBD
API Example is TBD
Removal of ID3 Tree class is TBD