initial commit of the merged POJ project

Previously, each problem has its own git repository, and it is really
difficult to manage so many repositories with GitHub. Therefore, the
individual repositories are removed, and all source files are merges
into this new central repo.
initial commit of prbl 1061

Created Makefile and date.c.
finish gcd
finish bezout (Euclidean algorithm)
finish modulus inverse
finish flinear congruence equation solver
finish whole program
fix negative solution bug

Previously, in the function a0 x = b0 (mod n), a0 is guaranteed to be
positive, but b0 was neglected. b0 also needs to be positive (actually
better in the range [1, n-1]) to assure that (x0 * b0) % n is positive.