Move out of the oslo namespace package

Move the public API out of oslo.context to oslo_context. Since this
library has not been released, we do not need to retain the old interface
for compatibility.

bp/drop-namespace-packages

Change-Id: I19adf862596abae0b034e81472114093425782f8
Generate better documentation for the module

Change-Id: Ibad798b09ac1227065173b3b1843cc3b6bfbaf80
Better information in the README

Change-Id: I51f8d6626ce1623f043a896cd0cc82478512f908