lisa: PerfAnalysis: cleanup

* Move to lisa.analysis.rta
* Give "action" name to methods where appropriate ('get_' prefix)
* turn tasks() into a property
* Streamline __init__ so it takes the common denominator between all
  ways of building the object.
* Add alternative factories:
    * from_log_files
    * from_task_names
    * from_dir

    note: from_dir() has a similar behavior to the former constructor.

* Rename camelcase methods to PEP-8 naming style like the rest of the
  code
* Remove leading double undescores from method names to avoid name
  mangling.
* Split plot_perf into 3 separate plot methods, like it is done in other
  analysis classes
* Remove dead code here and there
* Use pandas.read_csv instead of read_table to avoid FutureWarning

Changes to AnalysisBase:
* Split into AnalysisHelpers and AnalysisBase
* Improve AnalysisHelpers.save_to() to infer the image format from
  filepath.