Add search defaults to simpleldap.Connection class

Adding these search defaults makes searching as simple as it should be:
all that is required in client code is the LDAP filter string, when
given a connection.

After all of the tedious setup that is required for making the initial
LDAP connection (which simpleldap takes care of), my next biggest
irriation with using the LDAP API is having to continually pass in the
parameters which rarely change with the same connection, such as base
DN. This results in extra global context that needs to be passed in to
otherwise self-contained code.
debugging probably does not belong here.
Merge pull request #1 from elliotschlegelmilch/master

debugging probably does not belong here.
Rename method to set_search_defaults
Add test for existing reset_search_defaults params
Change reset_search_defaults to clear all w/no arguments
Rename reset_search_defaults to clear_*
Corrected typo in docstring.
Added search_defaults parameter to Connection constructor.
Made use of .clear() method to clear search defaults dictionary, and tweaked docstrings.
Merged search defaults feature, refs #2.  Thanks to Wil Cooley (wcooley) for the patch.