Updated the session library to handle multiple, named sessions.

- Includes passing unit tests.
Adjusted specs to only test multiple sessions for server connections (instead of embedded).

Embedded might be possible as well, but is outside my area of expertise.
This change should let the travis build pass.
Added an exception if trying to use multiple sessions with an embedded db, and updated the README.
Merge pull request #40 from undergroundwebdesigns/adding-multi-session-support

Updated the session library to handle multiple, named sessions.