Set Java compiler default encoding to UTF-8

[matt9ucci]

PR description

allprojects {
  tasks.withType(JavaCompile) {
    options.encoding = 'UTF-8'
  }
}

Fixed Issue(s)

When running the build script on Windows 10, the following warnings are displayed:

PS> .\gradlew.bat build -x test
Starting a Gradle Daemon (subsequent builds will be faster)

> Task :crypto:compileJava
C:\pantheon\crypto\src\main\java\tech\pegasys\pantheon\crypto\SECP256K1.java:141: error: unmappable character (0x88) for encoding windows-31j
    // routine specified in Section 2.3.7, where mlen = 竚?(log2 p)/8竚? or mlen = 竚?m/8竚?.
                                                         ^
C:\pantheon\crypto\src\main\java\tech\pegasys\pantheon\crypto\SECP256K1.java:141: error: unmappable character (0x89) for encoding windows-31j
    // routine specified in Section 2.3.7, where mlen = 竚?(log2 p)/8竚? or mlen = 竚?m/8竚?.
                                                                     ^
C:\pantheon\crypto\src\main\java\tech\pegasys\pantheon\crypto\SECP256K1.java:141: error: unmappable character (0x88) for encoding windows-31j
    // routine specified in Section 2.3.7, where mlen = 竚?(log2 p)/8竚? or mlen = 竚?m/8竚?.
                                                                                  ^
C:\pantheon\crypto\src\main\java\tech\pegasys\pantheon\crypto\SECP256K1.java:141: error: unmappable character (0x89) for encoding windows-31j
    // routine specified in Section 2.3.7, where mlen = 竚?(log2 p)/8竚? or mlen = 竚?m/8竚?.

These warnings are caused by windows-31j, default Japanese encoding of my PC.

The similar kind of warnings/errors are likely to happen in the other language environments. It will be nice to set the default encoding explicitly.

[ajsutton]

LGTM. Thanks.