Cleanup Elasticsearch formula and add the option to install `HEAD` version

### 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.

### Allow to install Elasticsearch from the source ###
    
        $ brew install elasticsearch --HEAD