Add a teal lexer

[iscke]

This PR adds a lexer for teal (https://developer.algorand.org/docs/reference/teal/specification/), the language used for smart contracts for the algorand blockchain.

I've tried to follow the coding standards used elsewhere in this project as closely as possible, but if anything's wrong please don't hesitate to let me know. Hope this can help.

[Anteru]

Thanks for the contribution! Please change the tests to use the new test system using golden files, it makes it easier to maintain. (See Contributing.md for details.)

[Anteru]

Thanks! A few more things: You have to move all examples into /examplefiles/teal (i.e. also tests/examplefiles/example.teal), and it looks like you're missing the golden test files (you can generate them using py.test --update-goldens)

[iscke]

oops, thank you

[Anteru]

Thanks. Just to make sure, 7777777777777777777777777777777777777777777777777774MSJUVU is really supposed to be a single number?

[iscke]

Yeah, it's a crypto account address

[Anteru]

Merged, thanks!

[iscke]

Thank you!