Gastric Carcinoma and Chronic Gastritis: Epigenetic Regulation of Cdh1 (E-Cadherin), Cdkn2A (P16 Ink4A), Ptgs2 (Cox-2) and Egfr Genes Through Methylation [Carcinoma Gástrico e Gastrite Crónica: Regulação Epigenética Por Metilação dos Genes Cdh1 (Caderina-E), Cdkn2A (P16 Ink4A), Ptgs2 (Cox2) e Egfr]