Add 'avg' aggregate function

The 'AVG' aggregate function is specified in the SQL standard and
supported by MySQL, PostgreSQL, and SQLite. Include tests, too.

lib/builder.js

@@ -372,6 +372,11 @@ _.extend(Builder.prototype, Common, {
     return this._aggregate('sum', column);
   },
 
+  // Retrieve the avg of the values of a given column.
+  avg: function(column) {
+    return this._aggregate('avg', column);
+  },
+
   // Increments a column's value by the specified amount.
   increment: function(column, amount) {
     return this._counter(column, amount);

test/integration/builder/aggregate.js

@@ -8,6 +8,12 @@ module.exports = function(knex) {
 
     });
 
+    it('has an avg', function() {
+
+      return knex('accounts').logMe().avg('logins');
+
+    });
+
     it('has a count', function() {
 
       return knex('accounts').logMe().count('id');

test/integration/output/Aggregate.js

@@ -22,6 +22,29 @@ module.exports = {
       }]
     }
   },
+  'has an avg': {
+    mysql: {
+      bindings: [],
+      sql: 'select avg(`logins`) from `accounts`',
+      result: [{
+        'avg(`logins`)': 1.6667
+      }]
+    },
+    postgresql: {
+      bindings: [],
+      sql: 'select avg("logins") from "accounts"',
+      result: [{
+        avg: '1.6666666666666667'
+      }]
+    },
+    sqlite3: {
+      bindings: [],
+      sql: 'select avg("logins") from "accounts"',
+      result: [{
+        'avg("logins")': 1.6666666666666667
+      }]
+    }
+  },
   'has a count': {
     mysql: {
       bindings: [],