factor out rvm_system_path and rvm_user_path, and fix minor path bugs

This removes duplication of some hardcoded values and paves the way
for adding support for mixed mode installations.

Remove trailing '/' from "$HOME/.rvm/", for consistency with default
system path "/usr/local/rvm".

Finally, in the corner case that rvm_type is set to a non-standard value,
rvm_bin_path will end up being the bin/ subdirectory of rvm_path,
rather than the same directory.
make default value of rvm_gemset_path depend on rvm_type

This paves the way for adding support for mixed mode installations.
add support for mixed installation mode via new :rvm_user parameter

rvm_user parameter will trigger running 'rvm user' with appropriate
arguments on the server.  It also helps determine the correct path for
importing/exporting gemsets, and when to use sudo (e.g. for gemset
creation/deletion only if rvm_user includes :gemsets).