* get/import/url: handle existing output directory
Currently if you run
```
dvc get https://github.com/iterative/example-get-started model.pkl --out .
```
it will nuke your cwd. To fix that we need to properly handle existing
directories, same as usual unix cli utilities do that.
* travis: specify python version when installing from chocolatey
Fixes#2609
* remote: drop not working http resume
Closes#2136
* remote: drop @transfer_context()
It is not needed for ssh anymore and GS/S3 are switched to client
instances in cached properties.
* remote: drop upload/download chunking
Ruslan Kuprieiev
Alexander Schepanovski
Paweł Redzyński
Ruslan Kuprieiev