move top-level Makefile to scripts directory

As noted in #194, it is now the default to compile with any
system-provided dependencies used by default. Move the top-level
makefile to scripts/. This Makefile is now a 'compile dependencies' file
which is intended to be a convenience for those that don't want to
install their own software :).

The 'scripts/' directory is a bit of a misnomer but I can't think of a
better name.
.travis.yml: update to handle moved scripts
README: update to reflect new default build style
build information moved to INSTALL.md

See comments in #194.