Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/failtest/ldlt_int.cpp

@@ -0,0 +1,14 @@
+#include "../Eigen/Cholesky"
+
+#ifdef EIGEN_SHOULD_FAIL_TO_BUILD
+#define SCALAR int
+#else
+#define SCALAR float
+#endif
+
+using namespace Eigen;
+
+int main() {
+  LDLT<Matrix<SCALAR, Dynamic, Dynamic> > ldlt(
+      Matrix<SCALAR, Dynamic, Dynamic>::Random(10, 10));
+}