Benchmark tool improvements

* support for benchmarking the keyword search
* added new options: 
  - minclients: minimum number of clients, useful when only interested in high load, e.g. fully saturate KWS and when benchmark DAS homepage
  - repeat: repeat each "job" (particular # of clients) multiple times to get better results
* support other output formats than pdf, such as png (taken from filename)
* code cleanup: pep8