Test membership check without posixGroup support

This blows up for some LDAP servers that don't understand the memberUid filter term.
Extract posix_member_filter
Add posix_support flag, default true, use in Domain#membership

Now we conditionally add the memberUid filter into the membership check
when posixGroup support is enabled only (the default).
Use the right instance variable
Correct local variable name
Test disabled posixGroup support
Bump version to 1.3.2
Merge pull request #38 from github/configurable-member_filter-memberUid-support

Configurable posixGroup membership filter support