Add Support for PKCE (#120)

- Move officially supported versions to Net45 and NetStandard2.0
- Migrate from VS2015 to VS2017 build system
- Move all build targets into single CSProj file
- Add support for PKCE oauth flow
- Add PKCE Oauth Example
- Add System.Security.Cryptography as dependency

(49cee186b12822462483dbca08dfb1335f55efbf)
Files Namespace
  - Add internal_error to SearchError union.
  - Add locked to LookupError union.
  - Add cant_move_into_vault to RelocationError union.
  - Add MoveIntoVaultError union.
  - Add SearchMatchFieldOptions struct.
  - Add optional match_field_options to SearchV2Arg struct.
  - Doc/example changes.
Sharing Namespace
  - Add is_vault to SharePathError union.
  - Add invalid_shared_folder to AddFolderMemberError union.
Team Namespace
  - Make members field of LegalHoldsPolicyUpdateArg struct optional.
  - Add app_folder_removal_not_supported to RevokeLinkedAppError union.
  - Doc/example changes.
Team Log Namespace
  - Add auto_approve to InviteMethod union.
  - Add moved_from_another_team to InviteMethod union.
  - Add moved_from_another_team to MemberStatus union.
  - Add no_one to SharedLinkVisibility union.
  - Add optional new_team to MemberChangeStatusDetails struct.
  - Add optional previous_team to MemberChangeStatusDetails struct.
  - Add external_sharing_create_report_details to EventDetails union.
  - Add external_sharing_report_failed to EventDetails union.
  - Add content_administration_policy_changed_details to EventDetails
union.
  - Add external_sharing_create_report to EventType union.
  - Add external_sharing_report_failed to EventType union.
  - Add content_administration_policy_changed to EventType union.
  - Add send_for_signature_policy_changed_details to EventDetails union.
  - Add external_sharing_create_report to EventTypeArgs union.
  - Add external_sharing_report_failed to EventTypeArgs union.
  - Add content_administration_policy_changed to EventTypeArgs union.
  - Add send_for_signature_policy_changed to EventTypeArgs union.
  - Add SendForSignaturePolicy union.
  - Add ExternalSharingCreateReportDetails struct.
  - Add ExternalSharingReportFailedDetails struct.
  - Add ContentAdministrationPolicyChangedDetails struct.
  - Add SendForSignaturePolicyChangedDetails struct.
  - Add ExternalSharingCreateReportType struct.
  - Add ExternalSharingReportFailedType struct.
  - Add ContentAdministrationPolicyChangedType struct.
  - Add SendForSignaturePolicyChangedType struct.

Author: rogebrd

.gitignore

@@ -1,4 +1,4 @@
-*.runsettings
+/dropbox-sdk-dotnet/Dropbox.Api.Tests/dropbox.runsettings
 
 ## Visual studio stuff
 *.suo