Fix compilation for latest vibe.d master. See also #10.

acquire/release/isOwner was removed recently.
Fix warnings. Fixes #11.
Remove the static assert to fix building with DUB and without vibe.d.

The compiler error won't be as clear in case of someone trying to use MysqlDB class now, but hopefully still clear enough.
Add back the static assert, but trigger it only if the MysqlDB class is actually used.
Minor doc change: Fixes #12
Minor cleanup: Bitfields are easier to read/verify in hex than dec.