Remove String manipulations of Date objects

This commit contains all previous work after rebase went badly
RE: Pull Request requested by @kjdelisle regarding #149 following my
proposed solution "A" at #149 (comment).
As mentioned in the post linked above, this change allows the
underlying mysqljs/mysql module to handle Date object serialization
and removes the forced conversion of Dates to UTC Strings.
An opt-out fallback to forced coercion to UTC is included,
which was modeled after #265 from @darknos .

Old, squashed commits:
d0ea1d9
Legacy UTC date processing fallback credit: @darknos

a59dad7
Remove orphaned string functions

e8fdbdc
Incorporate @darknos expanded check for zero dates

abd4e0a
Remove DATE manipulations in from/toColumnValue

lib/mysql.js

@@ -152,6 +152,13 @@ function generateOptions(settings) {
         options[p] = s[p];
       }
     }
+    // Legacy UTC Date Processing fallback - SHOULD BE TRANSITIONAL
+    if (s.legacyUtcDateProcessing === undefined) {
+      s.legacyUtcDateProcessing = true;
+    }
+    if (s.legacyUtcDateProcessing) {
+      options.timezone = 'Z';
+    }
   }
   return options;
 }
@@ -307,19 +314,6 @@ MySQL.prototype.updateOrCreate = function(model, data, options, cb) {
   this._modifyOrCreate(model, data, options, fields, cb);
 };
 
-function dateToMysql(val) {
-  return val.getUTCFullYear() + '-' +
-    fillZeros(val.getUTCMonth() + 1) + '-' +
-    fillZeros(val.getUTCDate()) + ' ' +
-    fillZeros(val.getUTCHours()) + ':' +
-    fillZeros(val.getUTCMinutes()) + ':' +
-    fillZeros(val.getUTCSeconds());
-
-  function fillZeros(v) {
-    return v < 10 ? '0' + v : v;
-  }
-}
-
 MySQL.prototype.getInsertedId = function(model, info) {
   var insertedId = info && typeof info.insertId === 'number' ?
     info.insertId : undefined;
@@ -356,7 +350,7 @@ MySQL.prototype.toColumnValue = function(prop, val) {
     if (!val.toUTCString) {
       val = new Date(val);
     }
-    return dateToMysql(val);
+    return val;
   }
   if (prop.type === Boolean) {
     return !!val;
@@ -417,8 +411,6 @@ MySQL.prototype.fromColumnValue = function(prop, val) {
         // those separate.
         if (val == '0000-00-00 00:00:00') {
           val = null;
-        } else {
-          val = new Date(val.toString().replace(/GMT.*$/, 'GMT'));
         }
         break;
       case 'Boolean':