Specify number column types (Fixes #316) (#317)

WaterlineJS defaults to using floating point field types for number
fields. The causes precision issues for file sizes since the reported
file sizes do not match the assets. This would also prevent the download
count tracking to work once the count is high enough such that one
download is beyond the supported precision.

Note: users must update their existing database schemas to avoid these
issues.

api/models/Asset.js

@@ -39,12 +39,14 @@ module.exports = {
 
     size: {
       type: 'number',
-      required: true
+      required: true,
+      columnType: 'integer'
     },
 
     download_count: {
       type: 'number',
-      defaultsTo: 0
+      defaultsTo: 0,
+      columnType: 'integer'
     },
 
     version: {