Enable ScramSHA1 auth provider

[tjyrkinen]

Fixes #35

Leaves mongocr as the default auth provider but allows setting ScramSHA1 via options: {authMechanism: 'ScramSHA1'}, identical to mongojs.

[nodkrot]

Need this! MongoLabs updated to MongoDB to v3. Please merge.

[gilbert]

This works! Here's an example of how to use it:

var connectionString = 'mongodb://...'
var db = pmongo(connectionString, {
  authMechanism: 'ScramSHA1'
});

[nicornk]

Please merge.

[imjul1an]

Is this fix is going to be merged?

[pihvi]

Works when you use in package.json dependency:
"promised-mongo": "git://github.com/tjyrkinen/promised-mongo.git#dist"