client region/metadata cache should have a public method for invalidating entries

While writing a testcase for HBASE-62, I observed that table metadata is cached as part of the region information cached client side. This cached region information (and therefore table metadata) is not directly invalidated by disable/enable table, so to get up to date metadata the client may have to use a scanner over .META. directly using the meta visitor. Ideally other client code – for example the support for HBASE-62 – should be able to invalidate entries as necessary, so then the next HTable.getTableDescriptor() would go to meta to return up to date information instead of incorrectly reusing outdated information from the cache.