Cubist

* A bug was fixed where the model failed if no predictors were included in the model (thanks to Gabe Gershenfeld for the find).

* The Windows build configuration was updated.

* Some files were synced with the C50 package.

* More efficient code was borrowed from the C50 package to write the data file to a string.

* A bug related to column names found by David Clifford was fixed.

* A bug was fixed that occurred when splits were determined in models with a categorical variable involving more than two categories (eg. "if X4 in {a, b}")

* Also, a bug was found in the variable importances calculations when a variable was used only in the conditions and not the model.

* The 'doc' directory was moved into the "vignettes" directory

* cubist uses a string buffer to capture the output files that would normally get saved to external files. The package was failing R CMD check only on windows because the output files were getting garbled. `s*printf` functions work differently on Windows. On Linux, they return the size that is needed to correctly print the string. On Windows, if there isn't enough space, they return -1.

* Also, work-arounds were added to the C sources because of R CMD check issues. `printf()`, `assert()` and `exist()` statements were removed or redefined.

* A bug was fixed for the `coef` method.