Include a proper license header in source files

[hegjon]

It would make it easier to redistribute multibit-commons if each source file contains a license header.

These files are without license headers:
./src/main/java/org/multibit/commons/concurrent/SafeExecutors.java
./src/main/java/org/multibit/commons/concurrent/SafeScheduledThreadPoolExecutor.java
./src/main/java/org/multibit/commons/concurrent/SafeThreadPoolExecutor.java
./src/main/java/org/multibit/commons/crypto/AESUtils.java
./src/main/java/org/multibit/commons/crypto/ECUtils.java
./src/main/java/org/multibit/commons/crypto/PGPUtils.java
./src/main/java/org/multibit/commons/files/SecureFiles.java
./src/main/java/org/multibit/commons/utils/Dates.java
./src/main/java/org/multibit/commons/utils/HttpsUtils.java
./src/test/java/org/multibit/commons/utils/DatesTest.java

Check https://bugzilla.redhat.com/show_bug.cgi?id=1331589#c8 for details