Dynamic thread pool for Spice Service, controlling core and max number of threads

Hi,

first of all, thank you for this incredible lib, it is very very useful for Android.

In something I am involved, we wanted to be able to configure the CorePoolSize and MaximumPoolSize on the Thread Pool Executor of the Spice Service.

I implemented this change on https://github.com/jorgevila/robospice/compare/stephanenicolas:master...dynamicThreadPool

creating two new methods getCoreThreadCount and getMaximumThreadCount that the user can override to modify these values.

I thought it could be interesting for you.

Best Regards.