perf(accounts): Use cached contact objects in accountForEmail

This method is called for every contact on a thread in the thread list, so I figured we should see if one version was faster. I ran this test code:

```
d = Date.now(); for(var ii = 0; ii < 10000; ii ++){ $n.AccountStore.accountForEmail("ben.gotow@gmail.com") }; console.log(Date.now() - d);
```

The other approach which calls meUsingAlias takes `3784ms`, and this version which uses the alias contacts cached in aliases() only takes `264ms`. Confirmed that the tests still pass.

spec/models/contact-spec.coffee

@@ -132,16 +132,9 @@ describe "Contact", ->
       c1 = new Contact {email: @account.emailAddress.toUpperCase()}
       expect(c1.isMe()).toBe(true)
 
-    it "also matches any aliases you've created", ->
-      spyOn(AccountStore, 'accounts').andReturn [
-        new Account
-          provider: "gmail"
-          accountId: TEST_ACCOUNT_ID
-          aliases: ["Ben Other <ben22@nylas.com>"]
-          emailAddress: 'ben@nylas.com'
-      ]
-
-      c1 = new Contact {email: 'ben22@nylas.com'}
+    it "it calls through to accountForEmail", ->
+      c1 = new Contact {email: @account.emailAddress}
+      acct = new Account()
+      spyOn(AccountStore, 'accountForEmail').andReturn(acct)
       expect(c1.isMe()).toBe(true)
-      c1 = new Contact {email: 'ben23@nylas.com'}
-      expect(c1.isMe()).toBe(false)
+      expect(AccountStore.accountForEmail).toHaveBeenCalled()

src/flux/stores/account-store.coffee

@@ -131,13 +131,13 @@ class AccountStore extends NylasStore
 
   # Public: Returns the {Account} for the given email address, or null.
   accountForEmail: (email) =>
-    @_cachedGetter "accountForEmail:#{email}", =>
-      _.find @accounts(), (account) ->
-        return true if Utils.emailIsEquivalent(email, account.emailAddress)
-        for alias in account.aliases
-          aliasContact = account.meUsingAlias(alias)
-          return true if Utils.emailIsEquivalent(email, aliasContact.email)
-        return false
+    for account in @accounts()
+      if Utils.emailIsEquivalent(email, account.emailAddress)
+        return account
+    for alias in @aliases()
+      if Utils.emailIsEquivalent(email, alias.email)
+        return @accountForId(alias.accountId)
+    return null
 
   # Public: Returns the {Account} for the given account id, or null.
   accountForId: (id) =>