GSoC: Pull request

Here is the promised pull request. I'm afraid I have touched every file and moved a lot of code around so that a diff will be somewhat hard to read, sorry for that. New features, summed up:
* BatchJobs backend
* bpmapply for all backends
* bplapply redirects to bpmapply
* bpaggregate, parallel version of stats::aggregate
* error handling: activated with catch.errors set to TRUE -> partial results, error messages and tracebacks are stored in LastError class
*  bpresume(expr) to resume previous call to bp[lm]apply with partial results

Yet there are still some minor issues:
* Package depends on BatchJobs not yet on CRAN, but will be uploaded in a few hours.
* the stored tracebacks are a little messy (functions for error handling taking most of the output).
* small documentation issues: I did not find out in time how to properly document bpaggregate with different signatures (formula and data.frame)
* Michael had some great ideas for future releases which did not made it into this version. I'll start implementing new features soon, using separate branches.

If you have any urgent changes to be made, or questions on the code, please ask.