List qtl as Depends:; change license to GPL-3; remove library(qtl) and require(qtl)

  - I think GPL (>=2) is too vague, as there are important differences
    between GPL-2 and GPL-3.
Changed addcov and intcov to addcovar and intcovar for consistency with R/qtl
In calls to scanone, specify pheno.col argument completely
Fix slight problems in examples
Add Makefile (for building Rd files) and .Rbuildignore file

 - Makefile is for creating the Rd files from the roxygen2 annotations
 - .Rbuildignore prevents Makefile and README.md from being included
   in the package (R wants README files in the inst/ subdirectory;
   Github wants them in the root directory)