[gtk-doc] Initial setup

Initial setup of gtk-doc. Straight forward setup following the gtk-doc
instructions. Ignore some troublesome types in src/hb-gobject.h. To
build use "./autogen.sh --enable-gtk-doc" then "make". Docs are in
harfbuzz/docs/reference/html/index.html.
diff --git a/.travis.yml b/.travis.yml
index a7285ae..82c4ecb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,11 +3,11 @@
   - clang
   - gcc
 install:
-  - sudo apt-get install pkg-config ragel   # for autogen.sh
-  - sudo apt-get install libfreetype6-dev   # for font functions
-  - sudo apt-get install libglib2.0-dev     # for font functions / tests / utils
-  - sudo apt-get install libcairo2-dev      # for utils
-  - sudo apt-get install libicu-dev         # for extra unicode functions
+  - sudo apt-get install pkg-config ragel gtk-doc-tools # for autogen.sh
+  - sudo apt-get install libfreetype6-dev # for font functions
+  - sudo apt-get install libglib2.0-dev # for font functions / tests / utils
+  - sudo apt-get install libcairo2-dev # for utils
+  - sudo apt-get install libicu-dev # for extra unicode functions
 script: ./autogen.sh && make && make check
 notifications:
   irc: "irc.freenode.org#harfbuzz"