Make path for cache file configurable

CVSAnaly relies on a directory in the home dir of the (system) user who executes cvsanaly.
This directory is used for cache file and config file if no other one is specified.
The path is built in [utils.py](https://github.com/MetricsGrimoire/CVSAnalY/blob/master/pycvsanaly2/utils.py#L148).

To get the path of a user is kind of problematic in python (os.environ.get('HOME')).
For example if you executes the process in a sub process (e.g. via [supervisor](http://supervisord.org/)), the wrong user is determined.

Situation:
* Supervisor runs as root
* Supervisor is configured to run the subprocess as user x
* Python script gets os.environ.get('HOME') => /root

Further more the user should chose the location of the cache file.
This pull request makes the file location configurable.