Add a `PhutilMethodNotImplementedException` class

Summary: I think that "Not implemented" is common enough to warrant a dedicated exception class.

Test Plan:
```
[2014-07-21 14:33:42] EXCEPTION: (PhutilMethodNotImplementedException) Method testMethod in class TestClass is not implemented! at [<phutil>/test.php:7]
  #0 TestClass::testMethod() called at [<phutil>/test.php:11]

[2014-07-21 14:33:14] EXCEPTION: (PhutilMethodNotImplementedException) Function foo is not implemented! at [<phutil>/test.php:13]
  #0 foo() called at [<phutil>/test.php:15]
```

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9992