resolve_before_precompile option

A configuration option for `BowerRails` was added that invokes `rake bower:install` and `rake bower:resolve` tasks before assets precompilation.

`Rake::Task['assets:precompile'].enhance ['bower:install', 'bower:resolve']` feature should be optional as it can break deploy process sometimes. Or it can be done manually. 

So I find this solution the most optimal and it can be merged if you agree. @rharriso, @jimpo, @romanenko