Added a basic Makefile.

Creates three binaries by compiling three different source files:
normal: E-vs-beta-Normal.cc
replica: Em-vs-beta-Replica.cc
mutualinfo: Mutual-info-vs-beta.cc

Example:
make normal will compile E-vs-beta-Normal.cc and generate the
executable "normal", which can then be run as ./normal and so on.

Special commands:
"make clean" will remove all generated binaries and .o object
files;
"make all" will compile all three source files and generate all
three binaries in one step.
Added usage comments to Makefile.