support platforms which does not have shasum command

use sha1sum command if there is no shasum command
Fix #172 on ZSH
Update install.sh

$HOME/.nvm used instead of introduced earlier NVM_TARGET.
Merge pull request #176 from witoldsz/patch-1

Update install.sh
Added instructions for the install script
Fixed variable expansion for `SOURCE_STR`
Merge pull request #177 from koenpunt/patch-1

Added instructions for the install script
Merge pull request #178 from koenpunt/patch-2

Fixed variable expansion for `SOURCE_STR`
Merge pull request #175 from Marsup/fix-172

Fix #172 on ZSH
Merge pull request #171 from cou929/support_sha1sum_command

support platforms which does not have shasum command
fixed wrong condition checking for 'shasum' (I'm using Mac OS 10.8.2)

line 188, `which shasum > /dev/null 2>&1` will be replaced by:

	'/usr/bin/shasum > /dev/null 2>&1'

but, `/usr/bin/shasum` needs filename argument which is ommited and the
test results always 'false'.
Merge pull request #181 from andrwj/master

fixed 'bash command not found sha1sum' in Mac OS
Update README.markdown
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.