Replaced custom Gradient with built-in Unity class

LibNoise.Unity was using an old custom Gradient class that is mostly
unnecessary now that Unity has its own. Replaced, keeping the old
definitions as part of new static class GradientPresets.
Updating README
Fix namespace scope errors for System.Math functions

Fix namespace scope errors for System.Math functions. Convert tabs to
spaces for consistency with other project files.
Add additional constructors that only require a module input
Add the rest of the module input only constructors & update README
Fix texture generators & add GetNormalizedData() function

Fix texture generators & add GetNormalizedData() function (useful for
generating height maps)
Update GetNormalMap() to calculate edge pixels

Added m_uncroppedData property to Noise2D, which contains a 1px border
of extra noise data used to calculate the edges of normal maps.
(Important for terrain tiling.) Also added an optional boolean to the
Indexer for setting/getting the uncropped data (useful for custom noise
map functions).
Simplify math in GetNormalMap() function
Add RGB gradient for generating splat maps & add "island" gradient for more natural coloring
Fix x/y coordinates in texture generators
Replace var keyword with type
Fix tiling bug
Fix isSeamless bool meaning & reword isCropped def
Fix RGB gradient and remove "island" gradient
Standardize code formatting
Update README
Convert README from txt to md
Update README
Update wording in README
Remove erroneous line breaks after [NonSerialized]