make javdoc read/write utf-8 & add meta tag with utf-8

Hi,

building grails-core fails for me with the error below because java doc generation does not use utf-8 at all stages. To make this consistent l added the necessary options to `build.gradle`.

    :grails-bootstrap:javadoc
    /Users/hauner/Development/Grails/grails-core.git/grails-bootstrap/src/main/groovy/grails/build/logging/GrailsConsole.java:669: error: unmappable character for encoding ASCII
     * Like {@link #userInput(String)} except that the user's entered characters will be replaced with ???*??? on the CLI,
                                                                                                       ^
    /Users/hauner/Development/Grails/grails-core.git/grails-bootstrap/src/main/groovy/grails  /build/logging/GrailsConsole.java:669: error: unmappable character for encoding ASCII
     * Like {@link #userInput(String)} except that the user's entered characters will be replaced with ???*??? on the CLI,
                                                                                                        ^
    /Users/hauner/Development/Grails/grails-core.git/grails-bootstrap/src/main/groovy/grails/build/logging/GrailsConsole.java:669: error: unmappable character for encoding ASCII
     * Like {@link #userInput(String)} except that the user's entered characters will be replaced with ???*??? on the CLI,
                                                                                                         ^
    /Users/hauner/Development/Grails/grails-core.git/grails-bootstrap/src/main/groovy/grails/build/logging/GrailsConsole.java:669: error: unmappable character for encoding ASCII
     * Like {@link #userInput(String)} except that the user's entered characters will be replaced with ???*??? on the CLI,
                                                                                                           ^
    /Users/hauner/Development/Grails/grails-core.git/grails-bootstrap/src/main/groovy/grails/build/logging/GrailsConsole.java:669: error: unmappable character for encoding ASCII
     * Like {@link #userInput(String)} except that the user's entered characters will be replaced with ???*??? on the CLI,
                                                                                                            ^
    /Users/hauner/Development/Grails/grails-core.git/grails-bootstrap/src/main/groovy/grails/build/logging/GrailsConsole.java:669: error: unmappable character for encoding ASCII
     * Like {@link #userInput(String)} except that the user's entered characters will be replaced with ???*??? on the CLI,
                                                                                                             ^
    6 errors

    FAILURE: Build failed with an exception.

    * What went wrong:
    Execution failed for task ':grails-bootstrap:javadoc'.
    > Javadoc generation failed.