Use a Redis-namespace by default

[kostia]

Keys used to store translations should be namespace by default.
Current state has 2 main problems:

• Programmer must know about internals and take care of potential conflicts.
• Programmer has to migrate her data to a namespaces setup.
• If a conflict happens, it's a pain-in-the-ass to debug.

[kostia]

Either the namespace prefix should be configurable, or the Redis itself (or even both).