Add experimental spline interpolation support

A new parameter named useSplines has been added. It is FALSE by default. This parameter makes use of splines rather than lines to connect the points on parallel axes. Only side-effect I can observe is that X axis is continuous now, which keeps us from seeing variable labels.
Merge branch 'pr/34' into splines

* pr/34:
  Add experimental spline interpolation support
added brackets
added the labels back to the x axis
added an example of the splines
Merge branch 'dev' into splines

* dev:
  remove null arguments in aes
  better stop description
  document
  add parameter descriptions
  stop checks for column labels
  use column labels where applicable and store in pMat obj
  do not allow 'funny' names in the mapping
  add label argument to ggally_diagAxis
  more curly brackets and white space
  added curly brackets to an if
change arg useSplines to splineFactor

allow for logical TRUE, FALSE, 0..n, I(0)..I(n)
fixed 'if' bug
whitespace
add constant column to data as transform is fussy. only use first value
add examples
updated docs on splineFactor
documented
added splines to travis
formatting for Rcheck
Merge pull request #39 from ggobi/splines

splines -> dev
added spline features to ggparcoord