2 New Classes for improved Multi Table features

[anubhav-narayan]

1. Added a Class _Con_SqliteDict which inherits SqliteDict but uses an existing Connection to initialize, it can be used for converting an existing sqlite connection to a SqliteDict connection, although it is not advised.
2. Added a Class SqliteTableDict which provides an interface for accessing multiple tables with a single object via nested dictionary syntax. It creates _Con_SqliteDict instances for its own connection internally to access tables.
3. Closes Issue #137

P. S. Added .db files to .gitignore

[anubhav-narayan]

Corrected Errors at line 276 with symbol CLEAR_ALL.