Librart Module split and Sonatype OSS maven repo support

I would like to make use of this tool as a library to produce picture subtitles from my easyjasub tool, to allow that I have:
- split the code into two modules: one containing the classes with the actual subtitle management logic and a top level one with the gui and command line tool related code
- added a common parent pom with the plugins and data required by Sonatype OSS maven repo
- added some license information in README.md which were present in 4.0 version but which I couldn't find outside of the .java files - this may be reverted in case

To split in two modules I just had to add some glue code for Logger and Core class as they were called statically. Also I have temporary add @Ignore the test BDSup2SubMainWindowTest, I couldn't get it to work when "mvn package" is run.

I would like to have BDSup2Sub uploaded to Sonatype OSS maven repo so that it can be easily used as a library, I can upload it myself it that's ok with you.