added execjs gem

added therubyracer gem
Merge pull request #61 from bonniecpk/js-runtime-error

added execjs gem
Merge pull request #62 from bonniecpk/refactor-analysis

refactored currency exchange investment analysis into a class
Refactored analysis methods into models
Added comments to spec_helper
Load factory girl when running spec
Added factories and spec for currency_ex analysis engine

Define Dummy factory in before :all