Added new library to handle fundamentals api.

[willthiswork89]

This start focused on the Earnings and overview API calls which are arguably the most important ones. More to come.

[ghost]

@willthiswork89 thank you for your contribution. Appreciate your help!

A few things to improve before I'll be able to include it in the next version:

1. Classes EarningsBase, EarningsParserBase, EarningsPeriodBase, OverviewBase, OverviewParserBase could be moved to AlphaVantage.Net.Fundamentals project, because they are not going to be used across other projects
2. Please add unit tests for FundamentalsClient. For that, you need to create AlphaVantage.Net.Fundamentals.Tests project under ./tests folder
3. Also worth to add demo in AlphaVantage.Net.Demo project and add corresponding section in README.md

[ghost]

@willthiswork89 also please add yourself as an author of the new AlphaVantage.Net.Fundamentals nuget package (in csproj file)