Add unit tests

[Zapata]

It will be much easier to maintain the tool if there is some unit test.

Examples of usefull tests:

• a test for each source that check that the code can compute the price. This will detect big errors like API changes and allow to quickly see which sources work.
• a test for each "Algorithm Based Asset" (HERO, HERTZ, ...)

[Zapata]

In my version, you can find this kind of tests:

• unit tests sources, ie: CurrencyLayer
• unit tests for assets with formulas, ie: HERTZ