remove .gitignore
This file does not belong in the repository, but rather locally. Everyone uses different tools, and may want to tell git not to track differennt files.
add GPL headers
use GCC's vector arithmetic
* elegant high-level coe to manipulate eval = (opening, endgame) as vectors.
* translates into fast assembly code, using SIMD instruction set.

No function change: printed out both PST into files and diff-ed them to verify.
compute PST dynamically:
* b->pst[] is updated by set_square() and clear_square().
* calc_pst(): calculated PST from scratch for a given color.
* verify(): verifies b->pst[] (and later whatever else we need to compute dynamically).

PS: if e1 and e2 are eval_t (vector of 2 int), one cannot write assert(e1 == e2);
The problem is that the binary operator '==' is also vectorized, so the result is a vector of bool, and assert() doesn't know what to do with it.
An extra function vector_equal() had to be introduced, in order to do that.
eval_t fix for Clang
doxygen documentation test