Fixes https://github.com/xanzy/go-gitlab/issues/285

* Add support for merge request: create todo

see https://docs.gitlab.com/ce/api/merge_requests.html#create-a-todo

* Add support for merge request: get diff versions

see https://docs.gitlab.com/ce/api/merge_requests.html#subscribe-to-a-merge-request
and https://docs.gitlab.com/ce/api/merge_requests.html#unsubscribe-from-a-merge-request

* update UpdateIssue (#6)

* Updated Issue struct and MergeRequest struct (#7)

see https://docs.gitlab.com/ce/api/merge_requests.html#cancel-merge-when-pipeline-succeeds

According to the API documentation, a `201 OK` will be returned
upon success for the block / unblock user call.

See https://docs.gitlab.com/ce/api/users.html#block-user

This commit adds the following badges

- the project License
- go report card score
- number of Projects go-gitlab is used by (sourced fom sourcegraph)
- number of open Github issues

It also rearranges them slightly.
Signed-off-by: Gavin-John Noonan <mail@gjnoonan.co.uk>

Ref: https://docs.gitlab.com/ce/api/merge_requests.html#list-project-merge-requests

> Note: the changes_count value in the response is a string, not an integer. This is because when an MR has too many changes to display and store, it will be capped at 1,000. In that case, the API will return the string "1000+" for the changes count.

* + ClosesIssues

* Fixed style
Add err check

* + CreatedAt

* +UpdatedAt

* + TimeStats to issue

* Change TimeStats to existed struct in issues receive

NewRequest() can potentially be called with a
nil value present in []OptionFunc. For example
this code would trigger a panic prior to this
commit:

  _, _, err := git.Users.ModifyUser(event.UserId, options, nil)

Following on to: https://github.com/xanzy/go-gitlab/pull/252

This MR exposes the SnippetsService on the client

- Centralize snippet definition in snippet.go

- Add support for description field on create, update, get for personal and project snippets

Since the response of `GET user/activities` contains a date format (`YYYY-MM-DD`) that cannot be unmarshalled into a `time.Time` automatically, additional logic was added to to cover the un-marshalling of the timestamps:

`UserActivityTime.UnmarshalJSON()`
`UserActivityTime.MarshalJSON()`
`UserActivityTime.IsSet()`

see https://stackoverflow.com/questions/25087960/json-unmarshal-time-that-isnt-in-rfc-3339-format

Thanks @lkysow for spotting these!

Target length for comments is around 80 chars.

* added _links property support in projects api

* support for _links in project api, godoc for Links struct

Project Snippets return `web_url` in the fields. Believe adding it to the struct here should allow us to pick it up.
Ref: https://docs.gitlab.com/ce/api/project_snippets.html