Parse LuaJIT version from LUAJIT_VERSION_NUM

- Parsing from LUAJIT_VERSION_NUM is more reliable than LUAJIT_VERSION
Make PDF Doc generation optional

- PDF documentation generation is now optional. Enable via
  ENABLE_PDF_DOCS=On
Merge pull request #202 from justfalter/fix_luajit_version_parse

Fix LuaJIT version parsing
Merge pull request #203 from justfalter/optional_pdfs

Make PDF Doc generation optional (disabled by default)
DNS spoofing for IPv6 addresses
Help / description in etter.dns head updated.
Rearrange comment
Replace TAB-whitespaces with spaces in etter.dns
Merge pull request #204 from koeppea/master

IPv6 support for dns_spoof plugin
Add travis-ci support :)
Merge pull request #207 from justfalter/add_travis

Add travis-ci support :)
Move clean_exit out of ec_main.c
Create libettercap

- Program code is now located within a shared library, libettercap.
Merge pull request #209 from justfalter/libettercap

First iteration of libettercap