Use Serializable transactions by default.

This follows the default behaviour of the official wrapper, and avoids a deadlock with simultaneous transactions when using the Truncate journal mode.
Ignore failure to set log callback.

SQLite logging will just not be available if setting the callback fails. (It must be set before any other use of the library, which may not be possible if some other component has already loaded SQLite in this process.)
Support 'real' as an alias for double.
Merge bugfix branch into master.

Conflicts:
	src/System.Data.SQLite/Properties/AssemblyInfo.cs
	tests/System.Data.SQLite.Tests/Properties/AssemblyInfo.cs