Ssh client for go-git

[alcortesm]

Continuation of #3.

From #3, @mcuadros said:

I dont understand how the your AuthMethods works, how I should set for example the credentials here:
https://github.com/src-d/go-git/pull/3/files#diff-93a872f722160e3c55c4194ccfb29664R57

BTW can you squash

It was my fault, I accidentally kept some fields private in the structs. My tests were all internal, so I didn't notice the problem.

I have fixed that, now it should be pretty easy to understand.

In any case, here is an example program that illustrates this new functionality. It is a program that tries to connect to a github repo by ssh:

• if a password is provided (not secure, this is just an example) then it tries password auth (not going to work, github does not accept password auth).
• if no password is provided, it tries to connect to a running ssh agent and let it resolve the authentication with the server.

Example of use:

; go-git-ssh-demo git@github.com:alcortesm/go-git.git
Successful Connection
; go-git-ssh-demo git@github.com:alcortesm/go-git.git asdfasdf
ssh: handshake failed: ssh: unable to authenticate, attempted methods [none], no supported methods remain
exit status 1

PS: I have also squash all the commits.

[codecov-io]

Current coverage is 68.83%

Merging #5 into v2.0.0 will not affect coverage as of 48d2d2f

@@            v2.0.0      #5   diff @@
======================================
  Files           13      13       
  Stmts          892     892       
  Branches       141     141       
  Methods          0       0       
======================================
  Hit            614     614       
  Partial         72      72       
  Missed         206     206       

Review entire Coverage Diff as of 48d2d2f

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