Added a few features; fixed a bug

I made a few changes. First of all, there was a bug with the abs function where it would convert to an integer (so abs(-4.5) would give 4.0.) This was because you used the C abs function (which works with ints) instead of fabs, its double equivalent.
I also added asin, acos, and atan as synonyms for the arc functions, an exp function, and a command-line option to only display the result without '=' for shell scripting and stuff.