Adding of RSA-SHA256 signature and oauth_body_hash parameter

[rvflash]

Hi,

First at all, thanks for your work on this package.

The purpose of this pull request is to add the following features:

1. Add the management of the signature method RSA-SHA256.
2. Add the management of the oauth_body_hash parameter in OAuth header. In order to not break the method's signature, a new one is created to manage it: SetAuthorizationHeaderWithBody.
   1- Fixes some golint errors.
3. Using of constants instead of plain text for header and parameters.
4. Using of context package instead of golang.org/x/net/context to remove external dependency. Go version required >1.7.

Thanks,

[rvflash]

Sorry for the late return, I also have limited time. As you want, I just wanted to help by decreasing the number of lint errors. Regarding the io.Reader, I will not argue but, rather than taking in strings or []byte slices, in this kind of usage, I use an io.Reader for data sources (work with every type).

[rvflash]

Hi Gary, gentle reminder :)