Switch to new slex based scanner.

I'd like a few more eyes on this before merging it into master..

I don't have any project that use ErlyDTL directly to test this on, so I've "only" used the test suite (and hoping it covers most aspects when it comes to the scanner), and all tests do pass with this new scanner.

I still need to fill in a bit more in the README for slex too..

When building, it won't regenerate the erlydtl_scanner.erl file unless you run `make slex`, in which case it will pull down slex into deps and build that first.

It does print a nice little notice in case the .slex file is newer than the .erl and you'd need to run `make slex`.
The idea is that only those developing with .slex files need the slex compiler on their system.

Conclusion

I don't know of any issues that needs resolving before this is ready to ship to a wider audience.
I will work on the documentation in the mean time.