Add readNumber(r io.Reader) function

I'm trying to keep things very low-level. I could have used `bufio` for
this, but as stated in the README the goal is to understand the basics
of Go.
Add readLine(r io.Reader) function

This commit breaks the build. It will be fixed in a next commit.
Add ReadBulk(r io.Reader) function

Note that this function considers that the input doesn't start with the
bulk identifier.

This commit breaks the build. Deprecated code will be removed later.
Remove deprecated code
Rename files
Parse status replies
Parse error replies
Change Parse signature to return interface{}

This will allow returning different types from this function.
Parse integer replies
Fix helper
Parse bulk replies
Rename helper function
Status and bulk replies are returned as string
Fix error returned
Parse multi-bulk replies

The implementation now is much easier than before, however there's still
room for improvement.

The test suite looks much more convoluted than expected, I might review
it later. I think all the possibilities are covered, though right now
I'm a bit tired to really check it.