Add an advanced R_environment.

Setup an R environment with a special R version:
<action type="setup_r_environment">
   <r_base  name="package_r_3_0_1" owner="bgruening" />
</action>

Allow downloading and installing an R package:

<r_package>https://github.com/bgruening/download_store/raw/master/DESeq2-1_0_18/BiocGenerics_0.6.0.tar.gz</r_package>
merge
Change the <r_package> tag to <package>, because we are already in a special <setup_r_environment> environment.
set the R_LIBS env var automatically
Merged in BjoernGruening/galaxy-central-bgruening/R_environment (pull request #219)

Implementation of the a r_environment to easily install R packages in a reproducible way.