Improve MongoDB support.

* Support configuration with MongoDB connection URI strings.
* Handle connecting to replica sets.
* Use standard notation database+collection rather than
  collection+sub_collection.

Note that these changes are NOT expected to be fully backwards
compatible for someone using mongodb with pysaml2 already.
_mdb_get_database: don't require tuple for URI
Merge branch 'master' of https://github.com/rohe/pysaml2
_mdb_get_database: Restore 'uri is db name' functionality.

Forgot to run the test suite after the last round of changes :(.
On the good side, this commit should actually restore backwards
compatibility with previous MongoDB support.
Merge pull request #43 from fredrikt/master

restore functionality