Adds ssh client to V2.0.0

[alcortesm]

SSH client for go-git.v2.

[mcuadros]

the third party packages goes at the end of the import list

[alcortesm]

Agree, TODO.

[mcuadros]

@alcortesm ping

[codecov-io]

Current coverage is 68.51%

Merging #3 into v2.0.0 will not affect coverage as of 6d30995

@@            v2.0.0      #3   diff @@
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  Files           13      13       
  Stmts          883     883       
  Branches       138     138       
  Methods          0       0       
======================================
  Hit            605     605       
  Partial         72      72       
  Missed         206     206       

Review entire Coverage Diff as of 6d30995

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[toqueteos]

Minor thing related to Go style, could we move the Err on all of these to the beginning? Examples:
https://golang.org/pkg/os/#pkg-variables, https://golang.org/pkg/net/http/#pkg-variables

[alcortesm]

I would love to do that, should I @mcuadros ?

[toqueteos]

LGTM

[alcortesm]

The current version does not implement the new AuthMethod functionality correctly and I have not understand what should the String() method does.

@mcuadros suggested I should take a look at ssh.AuthMethod, so all of them can be passed to the SSH client.

I have not fully understand all Máximo comments: I will need to think about it. Things will be much easier if the appropriate sections of go-git where documented.

[alcortesm]

Done.

But I still don't understand the purpose of having all this auth methods in go-git.client.ssh, there should be only one, that wraps a generic an externally generated []ssh.AuthMethod in a go-git.client.common.AuthMethod. Should we discuss about it?

[mcuadros]

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-93a872f722160e3c55c4194ccfb29664R57ç

BTW can you squash

[alcortesm]

I have gotten myself into a squashing hell, let's continue this PR in a new one.