Global Search for Documentation

This PR brings **global search to the documentation manuals:** type into the search box, and all manuals and documents are searched and results returned. The results also show which manual the result lives in.

*Staged at:* 
http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_build_and_typos/en/index.html

*Search for 'hadoop'; the results:* 
http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_build_and_typos/en/search.html?q=hadoop&check_keywords=yes&area=default

*Click on a link, and you see the search word(s) hi-lighted:* 
http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_build_and_typos/en/admin-manual/installation/installation.html?highlight=hadoop

To make this work, the top-level index.html—which use to redirect to the Developers’ Manual index.html—now is its own file, with most of the content of the Developers’ Manual moved up to the top level file.

Global search was implemented by merging the individual indexes created by Sphinx into one global index, using a custom Sphinx extension in the Sphinx configuration file. It is fairly agnostic, and should be extensible to other documentation sets. The Google Analytic codes are implemented in the top-level documents.