add a cache for parsed dates
compute data only if not in dry-run
retrieve the path extension only once
use sets data structures to optimize lookups
remove redundant exclusion check

check_methods are called twice for each hit. The first ones are sufficient to
decide if the hit should be excluded or not.
fix date key computation when timezone is missing
preserve compatibility with python 2.6

cbay reported that set comprehension was available only in python 2.7+.
This patch fixes the syntax to keep backward compatibility with python 2.6.
disable cache when OrderedDict is not available

Fallback to a non cached dates when OrderedDict is not available.
It can occur with python < 2.7 and Pypi OrderedDict is not installed.
add some documentation about PyPi OrderedDict
use a tuple ase the cache key instead of string concatenation

As suggested by cbay, the cache key can be a tuple instead of a string
concatenation.
add the date in the Hit attributes, forgotten during the merge of a work branch
silently fail if OrderedDict is not available for python 2.6
Merge pull request #300 from cbonte/perf-log-analytics

Optimizations for import_logs.py Fixes #5314

Kuddos for nice pull request! We hope to see more contributions from you in the future :)