Specify explicit UTF-8 encoding for compilation

UTF-8 is the default file encoding on some platforms (e.g. OS X), but
for other platforms (e.g. Windows), UTF-8 characters in classes like
ECKey cause errors during compilation on other platforms (e.g. Windows).

See http://pastebin.com/X9jEemje for examples of these errors.

This commit configures Gradle to explicitly use UTF-8 encoding for all
compilation tasks.