C++ wrapper for vte, version 0.20.4.

Current dependencies:
- to build a library from tarball:
  * vte >= 0.20.5
  * libsigc++ >= 2.2.3
  * glibmm >= 2.21.3
  * pangomm >= 2.24.0
  * gtkmm >= 2.17.2
- to build a documentation (that's why *mm dependencies are so high):
  * same as above and tools required by mm-common, that are:
    - perl
    - doxygen
    - dot
    - xsltproc
- to build in maintainer mode:
  * same as above and tools used by maintainer:
    - git (to generate a ChangeLog)
    - mm-common >= 0.6
    - recent autotools because no deprecated macros are used.
