Ui features (#6)

* fix #3: added h ui command to toggle ui
* fix #4: added -N flag to disable colors
* fixed styling of line numbers (-n)
* fix #5 add usage section
* add pkg/epub module readme
* add license terms to all file headers
* fixed usage printing
* added basic unit tests
* run the tests only on linux due to the use of base64 utitlity

pkg/epub/README.md

@@ -0,0 +1,21 @@
+# epub module
+
+This is a modified version of the [epub
+module](https://github.com/kapmahc/epub/). I fixed a couple of issues
+and I added code to retrieve the actual epub content, which the
+original module doesn't implement. Since the last update on that
+module was 9 years ago (as I write this), it doesn't make sense to
+send a PR. And it also makes no sense to me to publish it separately.
+
+
+# ORIGINAL README
+
+## epub
+A pure go implementation of epub file format.
+
+
+
+### Documents
+- <http://idpf.org/epub>
+- <http://www.cnblogs.com/Alex80/p/5127104.html>
+- <http://www.cnblogs.com/diligenceday/p/4999315.html>