fixes a bug: immutable empty statement lists (for sake of performance) becomes mutable on the fly
fixes incomplete solution of 27e6fcabc12470fc83d3ab38f998eab9b2098c68
fixes an exception handling bug

Pb: ConcurrentModificationException is silenced if thrown during processing

Solution: remove the catch

This code must have been written in response to a special case but we don't have any trace of it (in particular no test case at all)
added method #clear to ProcessingManager
Merge pull request #21 from cc18/addClearMethod

added method #clear to ProcessingManager
sandboxes APITest in a better way
specifies the exception throwing strategy for incorrect programs
specifies the exception throwing strategy for incorrect programs
removes usage of checked exception
adds a test about package visibility
improves .gitignore
improves and specifies the contract of--source-classpath (command-line) / setSourceClassPath (API)