Add mixed diagonal preconditioner to TRON method

Add mixed diagonal preconditioner to TRON method

We use (1-alpha)*I + alpha*diag(H_k) with
alpha=0.01 as a preconditioner to run PCG in TRON
method.

Following lines are the changes:
- Change TRON::trcg to TRON::trpcg with
additional argument M for PCG iterations
- Add l2r_l2_svc_fun::get_diagH and
l2r_l2_svc_fun::get_diagH to get diagonal
component of Hessian
- Add static function uTMv for weighted dot
for vector u and v with weight M

For detailed analysis, please check
http://www.csie.ntu.edu.tw/~cjlin/papers/tron_pcg/precondition.pdf