update several functions for submode awareness, and add `sbt-paste-region`

- add: comint buffer now has the buffer-local variable, `sbt:submode` holds the current _submode_ of the sbt buffer.
- update: `sbt-command` will return to the _sbt_ submode before executing a sbt command.
- update: `sbt-send-region` will enter console mode(Scala REPL) if sbt buffer is in _sbt_ submode.
- add: `sbt-paste-region` behaves like `sbt-send-region`, except that it uses paste mode of Scala REPL.
add `run-scala` command for easy launching of Scala REPL.

In addition,

- update `README.md` to introduce `sbt-paste-region` command
- preliminary support for _no-exit_ argument for `sbt-paste-region`
fix a bug that prevent to enter `:paste` mode
use `sbt:quit-paste-command` to send quit command

As Heikki Vesalainen suggested, sending "\004" directly is not a good
choice.  It seems that it doesn't work on Windows.  By introducing
`sbt:quit-paste-command` to hold whatever quit command for the system
(C-z for Windows, C-d for Unix), it would be little more portable.
Merge remote-tracking branch 'refs/remotes/hvesalai/master'

Conflicts:
	sbt-mode.el
Merge pull request #4 from cinsk/master

SBT submode awareness to several functions