Capability for ManyToOne, _ManyToOne_Rev, ManyToManyField relationsips is implemented.
Tidy up the macaron.py, Add test for complex selection
QueryString#delete() uses wrapper_clause mechanism

In QueryString#delete(), self.clauses["type"] == "DELETE" condition was used.
The new algorithm uses self.wrapper_clause.

i.e.)
self.wrapper_clause = DELETE FROM tbl WHERE pk IN (SELECT pk FROM (<SELECT SQL>))
<SELECT SQL> provides primary key values to delete. WHERE pk IN ... clause specify
the records and delete them.
Modify test/test_complex_selection.py
create_link_tables() will be called in create_table() by default

If off it, create_table(link_tables=False)
Change name of ManyToManyField to ManyToMany
Separate OpConverter class from QuerySet#select()
Tidy up OpConverter class by using Field#to_database() and OpConverter.CONV.
Add test for M2M relationship to test_complex_selection.py
Merge pull request #31 from nobrin/enhance-select-method

(close #29) Selection with INNER JOINed tables is implemented.