define formal methods for accessing the erb outvar

This adds methods for accessing the outvar and switches `capture_tag`
to use them.  They are tested implicitly b/c `capture_tag` is tested.
renaming the `CaptureTag` module to `Capture`

The set of "capturing" tools is now about more than just the
`capture_tag` method.  This is a better name to represent that.

No behavior changes.
adding a `capture` method for manually adding content to the outvar

This is a formal method for adding content to the outvar.  This should
be used with methods not invoked using `<%= ... %>`.  `capture_tag`
has been updated to use this.

It is tested implicitly via `capture_tag`.
`capture_render` capture method

This is a capture method that calls render and captures its output.
Use this to make render calls invoked via `<% ... %>`.
`capture_partial` capture method

This is a capture method that calls `partial` and captures its output.
Use this to make partial calls invoked via `<% ... %>`.
README updates for all the capture handling changes

Just adding documentation for all the capture changes.
Merge pull request #20 from redding/kr-outbuf

Capture handling improvemements