Add support for custom domains from My Domains

[ghost]

If a user sets up a custom domain using Setup > Company Profile > My Domain, there URL will be "https://abc.my.salesforce.com". Without this patch, the proxy calls will fail due to an invalid instanceUrl value of "https://my.salesforce.com".

[ghost]

Steps to reproduce:

1. From the Setup menu, go to Company Profile, then My Domain
2. Enter a value into the domain box and click the I Agree box and click the Register button
3. After the domain is setup, create a Visualforce page with the following script

<apex:includeScript value="{!URLFOR($Resource.static, 'static/jquery-1.5.2.min.js')}"  />
<apex:includeScript value="{!URLFOR($Resource.sample, 'forcetk.js')}"  />
<script type="text/javascript">
var successCallback = function() {
    console.log('success');
};
var failCallback = function() {
    console.log('failed :-(');
};

var client = new forcetk.Client();
client.setSessionToken('{!$Api.Session_ID}', 'v27.0');
client.query('SELECT Id, Name FROM Account ORDER BY Name LIMIT 20', successCallback, failCallback);
</script>

1. Open the Javascript console to see the network request (Network panel in Chrome)
2. Open the VF page with the above

Notice the request will fail due to the SalesforceProxy-Endpoint being set to "https://my.salesforce.com/services/data/v27.0/query?q=..."

[metadaddy]

Looks great - many thanks!