Update Node.js GAX package generation

[jmuk]

• Modify the problems around generation of index.js files
• Fix package.json contents
• Some other changes

updates #97

[codecov-io]

Current coverage is 91.62% (diff: 75.00%)

Merging #114 into master will decrease coverage by 0.11%

@@             master       #114   diff @@
==========================================
  Files             3          3          
  Lines           714        716     +2   
  Methods         150        150          
  Messages          0          0          
  Branches        113        114     +1   
==========================================
+ Hits            655        656     +1   
- Misses           59         60     +1   
  Partials          0          0          

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[jmuk]

@swcloud, please take a look. I think I can ignore the coverage report failures -- 91.62% is still good enough.

[swcloud]

LGTM