Added options to yarp.rb formula

I added some options to configure the build of YARP.

I added:
- shared library support (on by default): to build with static libray pass --without-shared
- build with rpath support (off by default): to build with rpath pass --enable-rpath

By default now brew will build yarp with shared libraries (which is required by some software such as [WBIToolbox](https://github.com/robotology-playground/WBI-Toolbox)).
It is also possible to enable the linking and installation with @rpath. This option is disabled by default.