Allow HTTP Proxy to be configured

[poornas]

fixes #273

[krisis]

@poornas,

1. Why are we not adding a new (overloaded) constructor which takes the proxy as a parameter?
   If it is exposed as a member of MinioClient object, isn't it susceptible to data race like proxy being modified while a PutObject of a large object is ongoing? i.e UploadPart API calls may be sent over the proxy while the CompleteMultipartUpload may be sent over the updated proxy. This causes unnecessary confusion.
2. We should update the README with a section describing how and when to use the proxy setting in MinioClient.

[poornas]

@krisis, Proxy is set only once during initialization of the underlying RestSharp client, and hence the data race scenario you stated above will not arise. This doesn't have to be added to README imo, because it is not per se a minio-dotnet sdk feature.

[krisis]

@poornas, I understand why there won't be any data race like I thought earlier. Thanks for clarifying that. Won't it help our users if we added an example that demonstrates how and when to set proxy client?

[poornas]

@krisis, added an example and changed it to a chained constructor.

[krisis]

Neat!

[sinhaashish]

LGTM