Restore recursive mkdir on LOG_DIR

Revert part of 2bcd901a9e60f24a4c1c972bc3354969a8c8258e.
Up to three levels of directories may need to be created, if a user only
defines STORAGE_DIR and uses the --instance parameter.

Now, if you want every subdir under LOG_DIR to be owned by USER, you
must create all but the last directory beforehand and give it the
correct owner. Automatically chown()'ing each level of directory created
would have been too complicated.

Fixes #199.
do not log OSError from os.makedirs, when the dir already exists
check against errno.EEXIST
carbon.conf comment clarification

MAX_CACHE_SIZE unit was not clear.
Merge pull request #206 from ImmobilienScout24/fixed_mkdir_logging

do not log OSError from os.makedirs, when the dir already exists
Add filter against metrics with misplaced dots in blacklist example

This helps preventing issue #181
Merge pull request #230 from g76r/f_doubledots

Add filter against metrics with misplaced dots in blacklist example
Merge pull request #202 from jamesjuran/fix_log_mkdir

Restore recursive mkdir on LOG_DIR
Document the fact that one can use regexps in the aggregation-rules.