Allow configuring weights

Sometimes, you might have servers with very different processing power. With no weighting, this will result in the entire run taking as long as the slowest server.

Allow weighting configuration with a comma-separated-value of weights:

```
$ export NOSE_DISTRIBUTED_NODES=4;
$ export NOSE_DISTRIBUTED_NODE_NUMBER=1;
$ export NOSE_DISTRIBUTED_NODE_WEIGHTS=3,1,4,1;
$ nosetests long_test_suit
```
