Use loglog scale for plotting performance data

This makes performance comparison easier, especially for smaller sizes.

An example:

![sort_perf](https://cloud.githubusercontent.com/assets/270503/5436349/24795482-8476-11e4-90be-c5faac6b68b3.png)

Here one may clearly see that there is no point in using GPUs when there are less than 1e5 elements to sort. It is also clear that all backends have similar algorithmic complexity for larger problems.

Compare it to the current plot:
![sort_time_plot](https://cloud.githubusercontent.com/assets/270503/5436421/17aa7fa0-8477-11e4-8d35-ad311b5f2e5d.png)