bundler support

[cmhobbs]

Add bundler to the project.

[maxjustus]

Big time. I'll try and set aside some time for this

[cmhobbs]

I've added bundler support to lib/sinatra-authentication.rb as seen in this commit: https://github.com/nilmethod/sinatra-authentication/commit/05b01aeb973897fa20ae812a719c7f15fe96e3a8

Does it make sense to add gems for all of the adapters supported to the Gemfile? I think that'd cause some unnecessary gems being installed on different systems. Should groups be established for gems pertaining to specific adapters? If so, how does that need to be handled when building the sinatra-authentication gem?