Add core build

Add a "core" browser build of ripple-lib which has a subset of features and smaller file size

**Restrictions**

+ No transaction submission
+ No orderbook tracking
+ No Account class
+ No SerializedObject class. The only implication for this build is that you won't be able to request binary account_tx

Using an unavailable class (e.g. calling `remote#createTransaction`) will throw an error such as `Error: Class is unavailable in this build: Transaction`

**Results**

```
$ du -k build/*
172	build/ripple-0.9.2-rc5-core.js
856	build/ripple-0.9.2-rc5-debug.js
832	build/ripple-0.9.2-rc5.js
328	build/ripple-0.9.2-rc5-min.js
132	build/sjcl.js
```