Add -s option to force install from source

Binary package support of `nvm install` is great. But in some legacy (but still widely used) systems, official binary packages don't work because of incompatibility of shared libraries like `libc.so`.

(for example, see http://midnightcodr.github.com/blog/2012/11/22/install-node-dot-js-v0-dot-8-14-onto-centos-5-howto/ )

If there were an option to force install from source, it would be handy for users of such systems.

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To support `-s` option, I have made following changes:

- added `-s` option in nvm.sh to force install from source.
- updated `help` message.
- updated README's *problem* section to mention about `-s` option.
- added tests for `install` with `-s` and binary installation.

thanks!