more information on skipped storage when applicable

returns `Skipped Storage: XXX` when a storage engine is not loaded.  For instance:

```
Brents-MacBook-Pro-2:oauth2-server-php brentshaffer$ phpunit -v
PHPUnit 3.7.28 by Sebastian Bergmann.

Configuration read from /Library/WebServer/oauth2-server-php/phpunit.xml

...............................................................  63 / 221 ( 28%)
............................................................... 126 / 221 ( 57%)
............SSS...SSS...SSS...SSS...SSS...SSS...SSS...SSS...... 189 / 221 ( 85%)
...SSS...SSS...SSS...SSS...SSS..

Time: 2.13 seconds, Memory: 12.50Mb

There were 39 skipped tests:

1) OAuth2\Storage\AccessTokenTest::testSetAccessToken with data set #3 (MongoDB)
Skipped Storage: MongoDB

/Library/WebServer/oauth2-server-php/test/OAuth2/Storage/AccessTokenTest.php:11

2) OAuth2\Storage\AccessTokenTest::testSetAccessToken with data set #4 (Redis)
Skipped Storage: Redis

/Library/WebServer/oauth2-server-php/test/OAuth2/Storage/AccessTokenTest.php:11

3) OAuth2\Storage\AccessTokenTest::testSetAccessToken with data set #5 (Cassandra)
Skipped Storage: Cassandra

# ...

39) OAuth2\Storage\UserCredentialsTest::testCheckUserCredentials with data set #5 (Cassandra)
Skipped Storage: Cassandra

/Library/WebServer/oauth2-server-php/test/OAuth2/Storage/UserCredentialsTest.php:11
OK, but incomplete or skipped tests!
Tests: 221, Assertions: 720, Skipped: 39.
```