Fix scheduling tasks with many deps/params.

Tornado limits maximum size of HTTP request (without body) to 65536 bytes by default. If there is a task with many parameters and/or dependencies, it could fail to be scheduled. Luigi server just closes RPC connection, logging this:
```
2014-08-27 08:40:37,728 tornado.general[2159] INFO: Unsatisfiable read, closing connection: delimiter <_sre.SRE_Pattern object at 0x27dd6a8> not found within 65536 bytes
```

This commit increases that limit to 2MB. "Two megabytes ought to be enough for anyone."