Make picante pass R CMD check

Current R CMD check performs more stringent tests on the import and
use of functions in external packages. A lightway method of passing
R CMD check is to explicitly refer to the NAMESPACE of these
packages, e.g., `quantreg::rq()`. A heavier alternative (in another
branch) is to change dependencies in DESCRIPTION and import these
functions, so that NAMESPACE would have lines like
```
    importFrom(quantreg, rq)
```
and to remove `require(quantreg)` in the function body.

Current R CMD check result is at http://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-debian-clang/picante-00check.html

This pull request contains a light-weight fix which adds `::`. Alternative suggestion that changes dependencies in DESCRIPTION and makes explicit `imporFrom` is available in another branch 
jarioksa/picante@d5cde4f81153c4e72544db154eb98d163244be78