convenience methods campaigns.by_name and by_id

hi sebastian,
had the need to have some convenient methods to retrieve campaign by name and id - 
also experienced some trouble getting json-1.4 built on my machine. so i bumped json to 1.8 

introduce campaigns.by_name and campaigns.by_id
rip obsolete include RR::Adapters...
adopt gemspec to reflect gem updates - bump version to 0.7
new GRNotFound exception, 
adopt unit-tests