Support QUOTA extension

This pull request adds 3 methods for QUOTA extension. Are you okay with the method signatures below?

**Method signatures:**
(common parameters: `quotaroot` = quota root name, `name` = box name)
* setQuota(quotaroot, quota, cb) - Sets quota to a quota root. `quota` = an object containing resource names and their limits as key-value pairs, e.g. { storage: 52428800, message: 1000 }. (I haven't tested this method yet since it requires some permissions.)
* getQuotaRoot(name, cb) - Gets quota roots of a mailbox. `cb` has 2 parameters: `err` and `quotaroots` which is an object mapping between each quota root name of the mailbox and usages/limits information of its resources. See example of `quotaroots` object below:

```
imap.getQuotaRoot('INBOX', cb);
quotaroots =
{
  INBOX: {
    storage: { // storage unit is KB
      usage: 229844,
      limit: 52428800
    }
  },
  'Another INBOX quota': {
    message: {
      usage: 100,
      limit: 1000
    }
  }
}
```

* getQuota(quotaroot, cb) - Gets quota information of a quota root. `cb` has 2 parameters: `err` and `quotaInfo` which is a usages/limits information of its resources.

```
imap.getQuota('Another INBOX quota', cb);
quotaInfo =
{
  message: {
    usage: 100,
    limit: 1000
  }
}
```