MySQL test cleanup

This is a follow-up to https://github.com/twitter/storehaus/pull/76

Build fails intermittently with the following error:
```
[info] ! MySQLStore.MySQLStore text->text: Falsified after 94 passed tests.
[info] > ARG_0: List("(,None)")
[info] + MySQLStore.MySQLStore blob->blob: OK, passed 100 tests.
[info] + MySQLStore.MySQLStore text->blob: OK, passed 100 tests.
[error] Failed: : Total 3, Failed 1, Errors 0, Passed 2, Skipped 0
[error] Failed tests:
[error] com.twitter.storehaus.mysql.MySQLStoreProperties
java.lang.RuntimeException: Tests unsuccessful
```
E.g. [this](https://travis-ci.org/twitter/storehaus/jobs/6360453) and [this](https://travis-ci.org/twitter/storehaus/jobs/6370545) 

I haven't been able to repro this on my machine - repeatedly calling `put ("",None)` and `sbt test`.

Making the following changes to catch this the next time travis build fails:
* Move multiget tests into separate tags
* Print expected and actual values on failures