Added the "gdm" and "gdtm" aliases.

* gdm = Stands for "git diff master". Compares differences between local, unpushed commits and origin/master.
* gdtm = Stands for "git difftool master". Same as above but using the default difftool.
Enhanced the "gld" alias format and colors.

* Author names use the blue color.
* Tags/branches use the light cyan color.
* Dropped the colon after author names and dashes after subject lines.