Cleanup elasticsearch

Cleanup of the Elasticsearch formula:

1/ Fixed incorrectly installed Sigar libraries on Mac OS X

   Move all neccessary files to `libexec/sigar`.

2/ Removed manipulating the bin scripts, use ES_HOME variable

   Instead of hardcoding paths to Homebrew locations in the `bin/elasticsearch`
   and `bin/plugin` scripts, set up the ES_HOME environment variable
   in the `bin/elasticsearch.in.sh` properly.

   Also, simplify the Regex for replacing path to Elasticsearch classes (`--classpath`)
   in the `bin/plugin` script and use the `ES_CLASSPATH` variable.

3/ Simplified elasticsearch.yml configuration

   Reorganize the commands, use `sub` instead of `gsub`.

4/ Cleaned up the caveats/info section

   Remove redundant information.
Added elasticsearch HEAD

Allow to install Elasticsearch from the source:

    $ brew install elasticsearch --HEAD

(You need a working Maven installation -- `brew install maven` to install it with Homebrew.)