Add _.drop, unit tests, and cleanup documentation for _.extend, _.defaults, and _.pick.

Former-commit-id: a45b0c45d52fdbe5f71984412d631f3dfe87965b

build/pre-compile.js

@@ -10,17 +10,20 @@
     'accumulator',
     'args',
     'arrayClass',
+    'ArrayProto',
     'bind',
     'callback',
     'className',
     'collection',
     'compareAscending',
+    'concat',
     'ctor',
     'funcClass',
     'funcs',
     'hasOwnProperty',
     'identity',
     'index',
+    'indexOf',
     'isFunc',
     'iteratee',
     'iterateeIndex',
@@ -97,6 +100,7 @@
     'delay',
     'detect',
     'difference',
+    'drop',
     'each',
     'environment',
     'escape',