Update README.md
grunt, not gruntjs
Links to Amber web site and repo
grunt vs grunt-cli, notes about global installation

Changes done adressing the following comments
https://github.com/amber-smalltalk/amber-examples/pull/49/files#r18026252
https://github.com/amber-smalltalk/amber-examples/pull/49/files#r18026307
https://github.com/amber-smalltalk/amber-examples/issues/50#issuecomment-56827964
amber-cli, not just amber
No request for manual installation of `grunt`

Details https://github.com/amber-smalltalk/amber-examples/pull/49#discussion_r18055043
local grunt requested in package.json
request for global grunt-cli

No details about local grunt installation as this is done by 
     npm install
Troubleshooting section added.
Update README.md

https://github.com/amber-smalltalk/amber-examples/issues/50
amber is found at two places
Merge pull request #49 from hhzl/master

listed grunt as tool, renamed sections