actually write README
fix colorspace comparison
various doc fixes
use constants as intended
optimize conversion if possible
avoid mandatory gamut mapping
hide more ctors and avoid duplicate state
some clarification
add perceptual difference calculation
clean-up of construction and conversion

Some ctors with ill-defined or superfluous semantics have been made
private or were removed. Also, conversion now has a unified
interface which simplifies tests.
more javadoc fixes and clarifications
add transposition
minor fixes
switch to immutable operation

The code was effectively immutable before, this is merely
to make it clear.
add generic checks on colors
streamline construction of PColor instances

The PColor.create() factory method may now be used to construct
PColor instances in a generic manner. Also, some convenience ctors
which duplicated PColors.create() functionality are removed.
declare version 1.1.0.0 in MANIFEST.MF
minor README edits