Context manager implementation

[uranusjr]

Also see: #24. I didn't implement "unload" because it could be pretty messy when there are nested or even interlaced apply-restore pairs.

With this implementation, users have two choices:

1. Use the context manager to mix different database engines

   from mosql.query import *
   from mosql import mysql, sqlite
   
   select(...)             # Standard
   with mysql.apply():
      select(...)         # MySQL style
      with sqlite.apply():
          select(...)     # SQLite style
      select(...)         # MySQL again
   select(...)             # Back to standard

2. If the user only wants to work with one specific database

   # These two lines is equivalent to the old "import mosql.mysql"
   from mosql import mysql
   mysql.load()

To compensate the absence of the "unload" functionality, "load" actually returns a dictionary of attributes that were replaced by it. Users can manually restore things back if they wish to.

from mosql import mysql
backups = mysql.load()

# MySQL style now

import mosql.util
for k in backups:
    setattr(mosql.util, k, backups[k])

# Back to standard

I have also added/modified some documentation and tests.

[moskytw]

This feature is implemented in a different way. I will release it in next version.