Clarification on hooks; warning on initial gemsets; gemset doc improved

Improvment over pull #211

Added a warning and a workaround to install gems to global gemset.

Custom after_install scripts mentioned, handy to pre-install some useful gems, since .rvm/gemsets/default.gems and .rvm/gemsets/global.gems are usually overwriten during rvm upgrade.

`--without-gems`, `--with-default-gems`, `--with-gems` flags documentation added to Installing and Upgrading pages.