Merge context package with the main package

[sagikazarmark]

Is your feature request related to a problem? Please describe.
Currently the stdlib context and gobdd context packages can only be used in one file together with import aliases.

Describe the solution you'd like
Merge the context package with the main package. The functionality belongs closely together anyway.

Describe alternatives you've considered
Create an interface in the main package (interfaces belong to consumers).

Related #83

Additional context
This could be a backwards compatible change: we could keep the original package and add a type alias pointing to the new type in the root package.

[bkielbasa]

the context was moved to his own package because I had problems with testhttp package. Go doesn't permit import cycles. We removed the testhttp package from the main repository so IMO we can go for it.