Merge branch 'master', remote-tracking branch 'origin'
Merge branch 'master' of github.com:liquibase/liquibase
CORE-1116 Allow for both "AND" and "OR" specifications of which contexts to run

Split out Contexts into Contexts (runtime context) and ContextExpression for filtering
CORE-1116 Allow for both "AND" and "OR" specifications of which contexts to run

ContextExpression can tak AND,  OR, ! and parenthesis operator
CORE-1116 Allow for both "AND" and "OR" specifications of which contexts to run

Additional tests
Merge branch 'master' of github.com:skelter/liquibase
Merge remote-tracking branch 'lblb/master'
null check required, backed by unit test
Merge pull request #268 from skelter/ContextNullCheck

Context null check