A Phase 2B, Randomized, Double Blind, Placebo-Controlled, Multicenter Study To Compare 6 Dose Regimens Of CP-690,550 Vs. Placebo, Each Combined With Methotrexate, Administered For 6 Months In The Treatment Of Subjects With Active Rheumatoid Arthritis Who Have Had An Inadequate Response To Methotrexate Alone