remove contrib/magit-classic-theme.el

I very much doubt that anybody is using this because it is very
incomplete.  If you do use it please contact me.

The removal is done with the okay of the original author of this
file.
Makefile: avoid meaningless warning on travis-ci
Name the ELisp install directory.
Factor out the install-info binary into a Makefile variable.
Makefile: use printf and ed instead of sed -i

sed -i is not POSIX
Makefile: generate magit-pkg.el on the fly
Makefile: cleanup variables
Makefile: add INSTALL.md to DIST_FILES
Makefile: sanitize path variables
Makefile: install libraries in a subdir of site-lisp
Makefile: move tarball targets to eof
Makefile: create separate target for script
Makefile: rearrange targets
Makefile: add CP, CPBIN, MKDIR command variables
Makefile: small fixes and cleanup
Makefile: create loaddefs less verbosely
set magit-version dynamically in git repo

Add new function `magit-version' and use it to set the variable
`magit-version' at the end of `magit.el'.  In a git repository this uses
`git-describe' to get a precise version string.  When Magit is installed
using `package.el' or the tarball a new file `magit-version.el' is also
installed, which is then used by the function `magit-version' since git
cannot be consulted.
Makefile: use same timestamp for all files in tarballs
Makefile: cosmetics
Makefile: replace underscore with dash in target names