Added ability to call APEX methods

We've been using this code here for a while now and I thought I would contribute it back.

Here are the APEX docs for Salesforce: http://www.salesforce.com/us/developer/docs/apexcode/

Here is some example usage:
```python
creds = {'username': 'my_user',
         'password': 'my_password',
         'security_token': 'abc12Trii',
         'sandbox': True}
sf = Salesforce(**creds)
payload = {"activity": [
    {"user": "12345", "action": "update page", "time": "2014-04-21T13:00:15Z"}
]}
result = sf.apexecute('User/Activity', method='POST', data=payload)
```