Fix overflow issue in DCT

Replace add/sub with their saturating equivalent to avoid overflow.
Should resolve #139
Check image size when reading targa file

Throw an error when image width or height is 0 when reading a targa file
Solves #140
Check range of integer values in PPM text file

Add checks in PPM text file reading to make sure values are within the
specified range.
Fixes #141
Add instructions for building from release tarball, no autoreconf required.
Remove unused variables

Fixes #142