adds image upload support from clipboard (upload to github)

on paste event (strg-v) the clipboard is checked for a image, if one is
found the user will be prompt for the image name with a pre entered name
(img_[unixtimestamp].png). the user can then enter either a name, accept
the given name, or abort (in which case nothing will happen).
image folder and branch are asked at github publish settings, commit
message can be configured in settings (below github commit message
settings), default commit message is 'Image upload.'
a missing / on the end of the given folder will be added if not given.

after a upload to github the raw.githubusercontent.com link will be
inserted. for example:
![](https://raw.githubusercontent.com/username/repo/imgBranch/imgFolder/imgName)

if the given image branch is gh-pages the github.io link will be
inserted instead. for example:
![](http://username.github.io/repo/imageFolder/imgName)
add's read support for githubProvider

adds 2 function to enable reading files from github Repositorys.
start stackedit with enabled githubPublisher

adds the opportunity to start stackedit with a injected publisher
(+downloader) so that it starts and loads a given file. This is enabled
by appending json formated text to the link. here is a example:

http://stackedit.io/editor#{"type":"github","publishAttributes":{"username": ... }}

example json string after the #:
{
        "type"              : "github",
        "publishAttributes" :
        {
                "username"   : "pete",
                "title"      : "myAwesomeMarkdownFile.md",
                "repository" : "myDocs",
                "branch"     : "master",
                "format"     : "markdown",
                "path"       : "path/to/my/text.md"
        }
}

"type" is the given provider type u want to access, someone could add
other providers if they like. if type "github" is given a fileDesc with
the given "publishAttributes" will be created and the file content will
be directly downloaded from github.
if the user has no github token (not logged in) he will be redirected to
the github auth page on editor start, and the download will proceed
after login.

if a file with the same title is already present inside stackedit, it's
content will be replaced with the content of the github file,
publishAttributes will also be overwritten (repo, branch, path, etc). so
be shure to not open a file where u have unpublished changes, because
these will be lost.

if a file which does not exists is given, an empty file will be opened, which
can be published to the given location.

- username   : github repository username (can differ from your username)
- title      : the file Title
- repository : github repository
- branch     : github branch
- format     : file format
- path       : github file path
- imgPath    : folder for image uploads (optional, see pull request #620)
- imgBranch  : branch for image uploads (optional, see pull request #620)
stackedit auto-download given file on startup

adds the opportunity to start stackedit with a url and title
so that it starts and loads a given file. This is enabled by appending
json formated text to the link. here is a example:

stackedit.io/editor#{"type":"download","importParameters":{"url": ... }}

json string after the #:
{
        "type"             : "download",
        "importParameters" :
        {
                "url"   : "http://url/to/my/file.md",
                "title" : "myAwesomeFile.md",
        }
}

"type" is the given provider type u want to use, someone could add
other providers if they like. if type "download" is given a fileDesc with
the given "importParameters" will be created and the file content will
be directly downloaded from the given url.
the title will be checked against all other loaded documents, if a file
with the same title already exists it's content will be overwritten by
the downloaded content.

- url   : the url where the file content can be downloaded
- title : the title which will be used
Merge branch 'deeplink_github' into imgUpload_and_deeplink

* deeplink_github:
  start stackedit with enabled githubPublisher
  add's read support for githubProvider

Conflicts:
	public/res/providers/githubProvider.js
Merge remote-tracking branch 'origin/deeplink_download' into imgUpload_and_deeplink

* origin/deeplink_download:
  stackedit auto-download given file on startup
fixed a error on checking image branch.
Merge branch 'image_upload_github' into imgUpload_and_deeplink

* image_upload_github:
  fixed a error on checking image branch.