Added the install family: `p_install_github`, `p_install_cran`, and `p_install_source`

Also added a checker to check the install type/location called `p_source_type`
made p_install work with github, CRAN and local zip.  This transfered (with tweaking) to p_load as well.  Now testthat fails because the output from p_load is more specific in the information provided (per Dason's comments in the code); we now know what failed to install, failed to load and loaded.
changed testthat to represent changes to p_load