Noise Documentation

Added documentation to the functions in noise.hpp.
Noise Includes

Reordered the #include directives in noise.hpp to put them in a sensible order.
Octave Fix

Fixed the Octave function in noise.hpp so that it no longer takes the arguments to forward through to the generator by universal reference.

As the generator is possibly invoked multiple times by a single invocation of Octave, the arguments to the generator cannot simply be forwarded through, as this might move them, meaning they may not be present in their original form when they're needed next.
Bias and Gain Improvements

Added guards to the Bias and Gain functions in noise.hpp so that they simply return their input when invoked with a bias/gain (respectively) of 0.5.

This avoids the overhead of calling pow/log for bias/gain values which would result in no change to the input.
Default Generator Includes

Put the #include directives for the default world generator into a sensible order.
Default Generator Macro Fix

Added a level of indirection to stringification in the default generator's macro definitions, so that macros are definitely expanded when they're stringified.
Default Generator - Rivers

Rounded out the rivers implementation.  Rivers now "blend" seamlessly into the ocean through the continental shelf.  Rivers now have a depth chosen by the "height" of the world -- in "higher" areas rivers will be shallower, in "lower" areas rivers will be deeper.

Also changed the type of depth-related parameters to an integer type rather than a floating point type.  This makes much more sense given the meaning of the value -- a fractional depth does not make sense in the context of Minecraft.

Removed the "river_depth" parameter as it is no longer needed.
Default Generator - Remove Debug Code

Removed old debug code from the default world generator.