Provide a better "inspect"

[kgilpin]

Working with the conjur API in a ruby repl (e.g. pry) is frustrating
because it seems to use ruby's default Object#inspect. This results in
objects being printed like the following (with large strings in place
of the ellipses):

# <Conjur::Role:0x007fbbf19323d0
@block=nil, 
@options= 
{:headers=> 
{:authorization=> 
"..."}, 
:username=>"..."}, 
@url= 
"...">

It'd be really helpful if #inspect returned a more concise and
descriptive string like #<Conjur::Role 'stripe:layer:layer1'>.