Overloaded PCA constructor and ( ) operator to implement Feature#2287 - PCA that retains a specified amount of variance from the data. A sample was added to samples/cpp to demonstrate the new functionality. Docs and Tests were also updated

Input/Output arrays are updated to support fixed-type and fixed-size semantic for all compatible types

a big patch; use special proxy types (Input/OutputArray, Input/OutputArrayOfArrays) for passing in vectors, matrices etc.

reworked nearly all of the OpenCV tests (except for opencv_gpu tests) - they now use the Google Test engine.