Rename remove() to delete() for consistency with PSR-16

[clue]

This simple PR renames the existing remove() method to delete() for consistency with PSR-6 and PSR-16. It also adds some documentation to be in line with #16 and #29, but the behavior is otherwise left unchanged.

// old
$cache->remove('foo');

// new
$cache->delete('foo');

Empirical evidence suggests that this method is rarely used and this release includes a number of BC breaks that aim at making this interface act more like a standard PSR-16 cache, so I figured it makes sense to rename this as part of this release.

[clue]

@jsor Good catch, updated!

[clue]

Rebased on master to resolve merge conflict now that #29 is in