Fix parser bug

After s, it always expected sin (same for cos, tan)
Rename Euler constant to eu
Add parsers for Abs and Signum
Better function table, width aligned fields

(thanks to Text.Printf)
Improved function table functions radically

At this point, a rather good-looking function table can be printed with
this call and looks like the below shown table:

ghci> defaultFunctionTable 0 40 10 (Fraction 1 (Variable 'x'))
              x       (1.0 / x)
--------------- ---------------
         0.0000        Infinity
        10.0000          0.1000
        20.0000          0.0500
        30.0000          0.0333
        40.0000          0.0250

We're also not using any third-party packages anymore for
pretty-printing but the standard Text.PrettyPrint.
Symmath, Release III.

Merge branches 'pu/simplify', 'pu/parse' and 'pu/funtable' into next
Two rules (Fraction, Power simplification)

1. Simplifies [0 / term] -> 0
2. Simplifies e.g. [exp(x) * exp(y)] -> [exp(x+y)]
Introduced explicit rules for Ln/Log