Refactored to support dynamic/multiple benchmarks

[oytuntez]

Today I refactored some of this package to support:

• multiple benchmarking classes, such as one for time and one for memory.
• custom benchmarking classes. This lets you benchmark almost anything, from network latency to database IOPS.
• show/hide specific columns from the result output.

This required the refactor of these files:

• AthleticEvent
• DefaultFormatter
• MethodResults
• A new directory: Benchmarkers
  • BenchmarkerInterface
  • TimeBenchmarker
  • MemoryBenchmarker

See my commits here: https://github.com/oytuntez/athletic/commits/master.

I didn't issue a PR as couldn't know how to organize the changes into tags/branches.