Add .npmignore

[Algunenano]

npm notice === Tarball Details === 
npm notice name:          camshaft                                
npm notice version:       0.61.10                                 
npm notice package size:  139.7 kB                                
npm notice unpacked size: 1.3 MB                                  
npm notice shasum:        9055bbc577dbd87d38a0e712bf6157afd7704dca
npm notice integrity:     sha512-O07OZQsx1ULNY[...]1IV//IHwZWWDg==
npm notice total files:   243                                     
npm notice 

As with the other projects, we are publishing much more than what is really needed.

cc/ @CartoDB/rt-managers

[rafatower]

$ npm pack camshaft
camshaft-0.61.11.tgz

$ tar -xzf camshaft-0.61.11.tgz

$ ncdu package

  968.0 KiB [##########] /reference                                                                                                                                                                                                                                                       
  528.0 KiB [#####     ] /test
  448.0 KiB [####      ] /lib
  136.0 KiB [#         ] /examples
   20.0 KiB [          ] /docs
   20.0 KiB [          ] /config
   20.0 KiB [          ] /tools
   16.0 KiB [          ]  CHANGELOG.md
    8.0 KiB [          ]  .jshintrc
    8.0 KiB [          ] /.github
    4.0 KiB [          ]  LICENSE
    4.0 KiB [          ]  HOWTORELEASE.md
    4.0 KiB [          ]  .travis.yml
    4.0 KiB [          ]  package.json
    4.0 KiB [          ]  Makefile
    4.0 KiB [          ]  README.md
    4.0 KiB [          ]  CONTRIBUTING.md
    4.0 KiB [          ]  .npmignore

The reference/ dir is actually its own package and could be .npmignore'd. In turn camshaft-reference should have its own .npmignore, I think.