Add installation target to the makefile

Add installation target to the makefile

Fixes #24

Defines and uses the $DESTDIR and $PREFIX variables. They can be
overriden from the command line. By default, they are set to "" and
"usr/local" respectively.

Defines and uses a $PKGCONFIG variable to set the location of the
pkgconfig configuration file (monocypher.pc).  That variable depends
on $PREFIX.

Copies libmonocypher.a, libmonocypher.so and monocypher.h to their
respective destinations, and creates the pkgconfig configuration file.