1) Changed Java source and target to 1.8
2) Added optional tsa argument to the jarsigner command
Signing JNLP file should not fail if jar with a main class already
contains JNLP-INF directory.
Added travis-ci configuration file.
Removed openjdk8 from the travis-ci build configuration
Tycho 0.21.0 will be used
Writer will be closed
Merge pull request #1 from lkoniecki/master

Enhancements to the tycho-jnlp-plugin