Created ImpalaDialect
Fixed ImpalaDialect to pass checkfile
Fixed typo in ImpalaDialect comment
Fixed check isDatabase for Impala
Refactoring of change a4f5736cbd36cdace1153b325abfd34f6daf5720. I've pulled down isDatabase into JdbcDialectImpl and have removed all other code that duplicated this feature. I have also turned off joins on Impala until they have better support for complex joins.

Conflicts:

	src/main/mondrian/spi/impl/JdbcDialectImpl.java