Several changes.
Using new class enum (c++11) and chnges made accordingly to this.

In class tetrisboard the filled rows field was removed and the public interface reflects this.

Some small AI in class Cumputer to.
Some unused code, may be used in the near future.
Moved the implementation of class Computer to ".cpp" file.
Great improvements of the Ai.
Some due to more calculations and some to better tuning of the parameters.