Support `--install` option on sourcing `nvm.sh`

Fixes #545.

Note that per http://unix.stackexchange.com/questions/5024/passing-a-variable-to-a-bash-script-when-sourcing-it-in-another-bash-script , `dash` and `sh` (and possibly other shells) do not support passing arguments to a script when sourcing - the `nvm_supports_source_options` method determines support for this, and the same method is used to bail out of the tests so that they don't fail on `dash` and `sh`. Yay for progressive enhancement :-)