Oral Clofarabine for Relapsed/Refractory Non-Hodgkin Lymphoma

* Each treatment cycle lasts four weeks during which time the participant will be taking study drug for the first three weeks only. Participants will be supplied with a study medication-dosing calendar for each treatment cycle. * Clofarabine is a tablet that will be taken orally in the morning once daily on days 1 through 21 of each 28-day cycle. Participants can receive up to a total of 6 cycles if they do not experience any unacceptable side effects and if their cancer does not get worse. * The following tests and procedures will be performed at specified intervals through out the treatment period: blood tests, physical examinations, vital signs, radiological exams and urine tests.