add configuration "grant_flows"

defaults to all grant flows + 'refresh_token'
Hint default grant flows in initializer template
check for available strategies before processing on endpoints
completely turn off disabled strategies on token endpoint (see note)

Note: I'm not sure whether this is necessary, as there is already a
validation in Doorkeeper::Request.
test that refresh_token request returns unsupported_grant_type error when it is not enabled explicitly.
Add reuse_access_token option
Remove unnecessary line
Undo unwanted rebase effect.
Use attr_reader for trivial reader methods.
Move the config closer to access_token_expires_in.
Add test for the default behavior without reuse_access_token.
Fix the wrong application of rebase.
Fix more rebase issue.
More on Ruby 1.9 style hash.
More on Ruby 1.9 style hash.
Move the find_or_create method to Doorkeeper::AccessToken model.
Doorkeeper::OAuth::Helpers is no longer necessary for find_or_create.
Merge pull request #295 from chitsaou/config-grant-flows

Only enable specific grant flows.
Releasing version 1.2.0.
Merge pull request #387 from kenn/reuse_token

Add reuse_access_token option