Always look up things in the 32bit portion of the registry.

Apparently, the MySQL installation (as of 5.6.21.1 MSI installer) will
create all registry entries in the 32-bit portion of the registry.
When run on a 64-bit R, one has to make sure to access that portion
explicitely, by requesting a 32-bit view of the registry.

Also see http://bugs.mysql.com/bug.php?id=42423.
Consider multiple candidate directories.

Current MySQL installers will install multiple components, and create
registry keys for several of these.  As a consequence, it is incorrect to
assume that any registry node under "MySQL AB" will have a "Location" key,
and even for those who do, some might be connectors or similar instead of
MySQL servers.

So we now postpone the selection of a single directory, and instead do some
more processing using a list of candidate directories.  We try to locate the
dll in one of these, and break if we succeed.
Changed locations of libraries.

At least if installed from mysql-installer-web-community-5.6.21.1.msi, the
locations of the libraries are now directly within MYSQL_HOME/lib, with no
lib/opt or bin dir involved.  I hope this works for other people as well,
otherwise we might have to set these variables dynamically somehow.
Always quote MYSQL_HOME variable for shell since it may contain spaces.
Merge pull request #16 from gagern/readRegistry

Better auto-detection of paths