DOC: add some links in install.rst

Author: Hyungtae Kim

doc/source/install.rst

@@ -299,9 +299,10 @@ Optional Dependencies
   :func:`~pandas.io.clipboard.read_clipboard`. Most package managers on Linux
   distributions will have xclip and/or xsel immediately available for
   installation.
-* Google's `python-gflags` and `google-api-python-client`
+* Google's `python-gflags <http://code.google.com/p/python-gflags/>`__
+  and `google-api-python-client <http://github.com/google/google-api-python-client>`__
    * Needed for :mod:`~pandas.io.gbq`
-* `httplib2`
+* `httplib2 <http://pypi.python.org/pypi/httplib2>`__
    * Needed for :mod:`~pandas.io.gbq`
 * One of the following combinations of libraries is needed to use the
   top-level :func:`~pandas.io.html.read_html` function: