ATC: Adding a draft synced version of the async ATCommand.send call.
Uptime: Adding a frequent call back code to visualize when the uptime stops.
AT Command: Added synchronous handling for slow commands. These are not very frequent but having them synchronous makes handling some tasks much easier.
Various changes.

* SMS : Added proper reception handling. Messages waiting to be read at startup are not properly handled.
* File navigation: Improved file listing (ls), added a "more" command
* Settings: Rewrote it to use PropertiesFile
* Improved PropertiesFile to add failsafe mechanisms (using temporary files for rewriting and backuping previous files)
* APN Autodetection: Removed some logs