basic auth support for gists

Allow setting GITHUB_USER and GITHUB_PASSWORD env vars, if
GITHUB_OAUTH_TOKEN is not set to use basic auth with the API.
rename gist filename: 'nopaste.txt' to 'nopaste'

The default filename for the gist API, 'nopaste.txt' (e.g. when reading
from STDIN) always comes out without syntax highlighting, due to the new
gist using the filename extension.

If we rename the filename to 'nopaste', without an extension, it uses
the shebang line, if available, to choose a syntax highlighting scheme.
Merge pull request #21 from rkitover/master

basic auth for gist API support