Some gtags related changes:

- added some key bindings for gtags;
- removed gtags from el-get packages list. Use the one that is installed with gnu global package
Added highlight-symbol extension
Added magit extension for convenient interactions with git
Initial configuration for timeclock and rect-mark  extensions
Added ace-jump-mode extension
Added switch-window extension for easy switching between buffers
Some appearance customizations

- Removed all extra UI components to get a distraction free mode
- Added the current data and time to the mode panel