improve startup.getValuation

[simpixelated]

TODO:

• [ ] dynamic user growth (#32)
• [ ] dynamic lifetimeUserValue (#31)
• [ ] unit tests (#7)

Links for reference:

• http://www.quora.com/Startup-and-Private-Valuations/How-do-investors-value-pre-revenue-companies
• http://www.forbes.com/sites/quora/2014/01/24/how-do-investors-value-pre-revenue-companies/
• http://www.caycon.com/valuation.php
• http://www.angelblog.net/Business_Valuation_-_What_will_your_company_sell_for.html
• http://berkonomics.com/?p=1214
• http://business.tutsplus.com/tutorials/how-to-calculate-the-value-of-your-early-stage-startup--cms-65
• https://clarity.fm/questions/120/what-is-the-average-series-a-funding-round-at-pre-revenue-valuation-for-a