Store price to BigDecimal

[henu]

How about storing price as BigDecimal? It's perfect type for price information, as you can avoid floating point rounding errors and have as many decimal places as you want.

I'm not an expert in developing Java libraries, so please suggest how to make this PR better!

I'm only able to test it with Android Google payments.

[henu]

Automatic tests found some problems. Now they are fixed, and waiting for new run...