Issue #340: better dev-setup.sh script and server start

* Abort the script if we don't have a config.ini file set up
* Check that the install folder is actually a git repo
* Download the TMX archive only if we don't have a web/TMX folder
* Download composer only if we don't already have it
* Install composer dependencies only if it is not already done
* Check if json files exist before creating them
* Rename stats.json as stats_locales.json
* Create a start.sh script at the root of the repo to set everything up and start the dev server
* New 'dev' variable in config.ini to indicate if we have a development install of a production install
Merge pull request #341 from pascalchevrel/issue-340-add_stats_request_json_file

Issue #340: better dev-setup.sh script and server start