Add winkerberos support

[RiviqVH]

I am unable to install dbt-impala on a Windows machine from pip, because of the dependency on the kerberos package, which cannot be installed on Windows.

In order to support dbt-impala on Windows, it would be nice to have the winkerberos package instead (see this thread).

[RiviqVH]

Note that my PR (#141) solves this issue by simply removing the kerberos requirement from the setup, but does not implement winkerberos in any way.

[csringhofer]

Note that there is a PR in impyla about using winkerberos:
cloudera/impyla#504

I was hesitant to merge it as I was unable to test it, but it seems safe to fallback to winkerberos if kerberos is not found. Doing this in impyla should allow dbt-impala to work with winkerberos.

[csringhofer]

fyi cloudera/impyla#587 was merged and is released in impyla 0.22.0
it doesn't install winkerberos, but if winkerberos is present while kerberos is not present, then it uses winkerberos