Check for presence of SHELL environment variable

- POSIX states the SHELL environment variable "... shall represent a
  pathname of the user's preferred command language interpreter." As
  such, st should check for its presence when deciding what shell to
  use; just as HOME can be defined to override one's passwd-defined home
  directory, a user should also be able to override their passwd-defined
  shell using the SHELL environment variable.
added .gitignore file
symlinked config.def.h to config.h
initial commit of my config.def.h
default font is now DejaVu Sans Mono for Powerline
switched to zenburn colortheme