GetOverdueLogNames(string log_directory, int days) will check all
filenames under log_directory, and return a list of files whose last modified time is
over the given days (calculated using difftime()).

So that we can easily for unlink all files stored in the returned vector.

In this commit, at the end of LogFileObject::Write,
it will perform clean up for old logs.

It uses GetLoggingDirectories() and for each file in each
directory, it will check if a file is a log file produced by glog.
If it is, and it is last modified 3 days ago, then it will unlink()
this file. (It will only remove the project's own log files,
it won't remove the logs from other projects.)

Currently it is hardcoded to 3 days, I'll see if this can be
implemented in a more flexible manner.

The log cleaner can be enabled and disabled at any given time.
By default, the log cleaner is disabled.

For example, this will enable the log cleaner and delete
the log files whose last modified time is >= x days
google::EnableLogCleaner(x days);

To disable it, simply call
google::DisableLogCleaner();

Please note that it will only clean up the logs produced for
its own project, the log files from other project will be untouched.

Also replaced the hardcoded overdue days with the correct variable.

Previously the full path to a file is passed into IsGlogLog(),
and then std::string::erase() is used to get the filename part.
If a directory name contains '.', then this function will be unreliable.

Now only the filename it self is passed into IsGlogLog(),
so this problem will be eradicated.

Splitting a filename into tokens by '.' causes problems
if the executable's filename contains a dot.

Filename should be matched keyword by keyword in the following
order:
1. program name
2. hostname
3. username
4. "log"