Allow Database#get to take an array to return multiple values, similar to map/select_map

This is very similar to the map/select_map change made some time
go.  For array arguments, we need to make sure we have a valid
alias for each array element, then we just use values_at to
retrieve the resulting values.

This could potentially cause backwards compat issues when using
a conditions specifier:

  DB.get([[:a, 1], [:b, 2]])

That type of code now needs to be updated:

  DB.get(Sequel.expr([[:a, 1], [:b, 2]]))

The same was true of the map/select_map change, but I didn't
realize the backwards compat issues back then.  However, as
nobody reported problems, it's unlikely anyone was relying
on the old behavior.
Remove explicit encodings set in the specs
Skip stripping of blob columns in the string_stripper plugin (Fixes #593)

In addition to handling blob columns/values, this also allows
you to arbitrarily skip stripping of certain columns via
Model.skip_string_stripping.
Better handle new rake/old rdoc combination in Rakefile

Instead of just rescuing just LoadError, rescue StandardError
exceptions as well. New rake raises a RuntimeError when
rake/rdoctask is required.
Make Database#schema_and_table always return strings for the schema and table

Before, it sometimes returned a string, and sometimes returned a symbol.

While here, update the documentation to reflect that this method
does not handle more than one level of qualification.
Add Dataset#split_qualifiers helper method for splitting a qualifier identifier into array of strings
Make Database#schema handle [host.]database.schema.table qualified tables on Microsoft SQL Server

Basically, the schema parsing queries need to use the [host.]database
as a qualifier of the metadata tables.  There are not currently any
adapter tests for this, but I've done some manually testing and it
should work correctly.
Explicit require rake < 10 on Travis

1.9.2 is currently failing due to the new rake, even with the
Rakefile addition to ignore errors when requiring rake/rdoctask.
Document foreign_key :name option in schema_modification.rdoc
Update some spec guards for HSQLDB 2.2.9

Looks like between HSQLDB 2.2.5 and 2.2.9, changes were made that broken
the correlated_subquery eager limit strategy, and an internal
java.lang.ArrayIndexOutOfBoundsException is now raised if you try to add
a self-referential foreign key.
Make HSQLDB use standard SQL datetimes

This isn't always necessary, but it appears to fix some cases
on HSQLDB.
Add date_arithmetic extension for database-independent date calculations

This is supported on all 11 database types that Sequel is
regularly tested against.

While here, fix an minor issue in the postgres adapter specs when
activesupport is not installed.
Handle Java::OracleSql::TIMESTAMPTZ on jdbc/oracle
Fix error message for mismatched number of association keys

It is not guaranteed that composite keys are being used,
so just drop the composite qualifier.
Add a better explanation in the many_through_many documentation
clean up test
fix binary data escaping for DB2
Minor cleanups for db2 blob commit

Use fewer String#<< calls in literal_blob_append.

When use_clob_as_blob is true, don't cast to blob.

Don't use let in the specs.

Remove jdbc/db2 guard for blob escaping.
update JDBC driver loading to handle latest jdbc gems
latest gems no longer auto-load the driver jar but instead provide a load_driver method, this change is handled backward compatible (since not all driver gems are updated + users might decide to stick with an older gem version).
Attempt to use the jdbc-hsqldb driver if available

Sequel only supports HSQLDB 2.2+.  Previously, the jdbc-hsqldb
gem was at version 1.8.1.3.  However, it has recently been upgraded
to 2.2.9, and Sequel now supports it, so load it automatically
so that the jar does not need to be loaded manually.