refs #1700 basic performance analytics: handle server generation time for each page and page title

CORE

 * formatting sub-second times
 * getColumn() method on data table array (in order to behave the same as the regular data table class)
 * data tables can store in their meta data, which columns are empty (this is used in order to dynamically hide the new "generation time" column)
 * ViewDataTable and Api.getProcessedReport act according to the empty column meta data


SCHEMA

 * new column custom_float_1 in log_link_visit_action
 * new version to apply the change


TRACKER

 * Piwik_Tracker::setGenerationTime
 * tracking parameter "generation_time_me"
 * value is stored in new custom_float_1 column
 * the log importer can handle a group "generation_time_micro" which can be used in a custom log format. _micro is used because apache can log the time in microseconds but piwik processes milliseconds.
 * note: extension of JS tracker still missing


ACTIONS PLUGIN

 * for pages and page titles, add new columns sum_time_generation and nb_hits_with_time_generation to the blob archives
 * if they are set, compute avg_time_generation on the API level. if not, remove the columns and mark them as empty in the data table meta data.
 * show new column "avg. generation time" in the pages and page titles reports


plus TESTS for everything