Have association_join work with existing model selections

association_join uses eager_graph internally, which uses
graph.  graph requires knowledge of the columns in order
to setup appropriate aliases.

Since association_join just wants the joins and ignores
the changed selection anyway, add a :join_only option to
graph that just does the joins without changing the
selected columns, and have the eager_graph procs use
that option.
Add note about using Model#refresh to get all values in class_table_inheritance plugin

Also add a spec to ensure it doesn't break.
Add documentation note about using symbols for column names on MySQL for certain schema changes

MySQL doesn't have the ability to change the column type, default,
or NULL/NOT NULL setting indepdently, it just has a CHANGE COLUMN
syntax that combines all three settings.  Sequel attempts to parse
the existing values from the schema, but that isn't going to work
unless the column is given as a symbol, since the schema hash is
keyed with a symbol.
Fix error in documentation (two=>to)

Documentation had "two" instead of "to".
Don't associate extension rdoc with Sequel module

RDoc doesn't differentiate file-level RDoc from RDoc for the first
module, so if you only have one RDoc comment block at the top
of the file, and then a class or module after it, the RDoc comment
will be used for both the file itself and the module.

Add empty comment above Sequel module so that the file level
comment isn't used for the Sequel module.
Recognize additional disconnect error in jdbc/jtds adapter
Fix some grammar issues in association basics guide (Fixes #867)

This is a squashed commit from gferguson-gd.
Merge branch 'patch-2' of https://github.com/Luiji/sequel
Handle composite primary keys when working around validation issues for one_to_(one|many) associations in nested_attributes plugin (Fixes #870)

When a model uses a composite primary key, pk is never nil.  The
previous code resulted in the foreign key in the associated object
getting set to an array, which could make validation of the
associated object fail.  This uses the 0 value temporarily in the
composite primary key case, just as it is used in the simple
primary key case.