Enable gradle into the project

Signed-off-by: Victor <[email protected]>

Check if given keys are null or empty

Check if given keys are null or empty, if it is the case a
BitsoApiException will be thrown.

Signed-off-by: Victor <[email protected]>

adding BCH_BTC book

link to apiv2 wrapper

test build.gradle

[maven-release-plugin] copy for tag bitso-java-2.0.9

[maven-release-plugin] copy for tag bitso-java-2.0.8

[maven-release-plugin] copy for tag bitso-java-2.0.7

[maven-release-plugin] copy for tag bitso-java-2.0.6

[maven-release-plugin] copy for tag bitso-java-2.0.5