bump to v2.1.15 with readme fixes and minor code optimizations

package.json

 {
     "name": "sqlite3",
     "description": "Asynchronous, non-blocking SQLite3 bindings",
-    "version": "2.1.14",
+    "version": "2.1.15",
     "homepage": "http://github.com/developmentseed/node-sqlite3",
     "author": {
         "name": "Development Seed",