added the description about -s option.
fixed syntax error.
added -s (install from source) option.
added test for installation with -s option.
minor modifications.
fixed the condition for the binary package availability.
changed shasums file url because in v0.8.6 .txt.asc file is not available.
Merge pull request #189 from kt3k/feature/hyphen-s-option-2

Add -s option to force install from source
merge nvm source code

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>