Autoindent for buffers, currently fixed at 2 spaces.

The current buffer is lexed using the default Groovy Lexer,
which keeps track of parentheses in a stack, using the depth of that stack.

Also had to use a hack-ish solution wrapping the InputStream because JLine
does not provide an API to do autoindent.
fix import completion, reuse JLine ClassCompletor, complete "as" fixed
keep as instance var so that other completors may benefit from search result
Also complete Classnames for preimported and imported classes

Example:
groovy:000> Sys<tab>

Before: no completion
Afer: completes to System  (from java.lang.System)

Also:
groovy:000> import java.awt.*
groovy:000> Sha<tab>

now completes to Shape