Add support for explicit import statements using @ContractImport
Add preliminary support for Java cross-compilation in build.xml

Support is currently incomplete due to the fact that Cofoja itself
does not provide settings equivalent to javac -source and -target, so
contracts are still compiled with the default settings.
Give more explicit names to JAR files
Default to bootstrap build path with freshly built bootstrap.jar

The targets have been renamed; 'dist' now refers to the old
'bootstrap', and the old 'dist' has been renamed 'nobootstrap'.
Replace README with new README.md

The new README file has been updated with current information,
including complete documentation for almost all the currently
supported features.