use alias_method instead of alias

alias is lexically scoped, so it doesn't work with inheritance because
scope is determined at require time instead of runtime.
alias_method uses runtime scoping so it will do the right thing when
inheritance is involved.
don't even try to test 1.x features on 0.90

This signals that certain tests are not being run with the test and
assertion counts.
test method aliases

We should test method aliases so we don't inadvertently break them. Some
tests were slightly rearranged.
add an mget alias for multi_get

For consistency with msearch/multi_search. The name of the action in the
url is `mget` so it could be useful for people who don't read the docs.
Merge pull request #90 from github/alias-method

use alias_method instead of alias