Fixed closed cursors

Closed cursors should raise InterfaceError
Merge pull request #36 from VFLashM/master

bit-null added to NUMBER type
Merge branch 'fix-closed-cursors'
Added tox.ini

Added python3.4 to supported Pythons list
Removed python2.6 from supported Pythons list
Updated link to django-sqlserver backend
Improved readme
Added set_autocommit method

For compatibility with ADO and django-mssql
Small refactoring and docstrings
Removed TestLoadBalancer

LoadBalancer is now deprecated
Fixed tests on MacOS X

Added EPIPE to expected connection errors
Refactoring
Added docstrings
More docstrings
More docstrings
Refactored state_context method to use contextmanager decorator

And more docstrings
Refactored state_context renamed to querying_context.
Cleanup, removed query_flush_packet method
Fixed (n)varchar(max) writing for empty strings

Also produce only (n)varchar(max) for parameters, get rid of size condition
Simplified tests
Removed make_varchar and make_nvarchar methods, cleanup
Always generate long_binary in make_param, don't generate VarBinary(x)

Simplifies code, removes unnecessary condition.