Support for joins - #28

Pull Request Test Coverage Report for Build 78

• 376 of 421 (89.31%) changed or added relevant lines in 7 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.0%) to 82.217%

Totals
Change from base Build 74:	1.0%
Covered Lines:	2492
Relevant Lines:	3031

💛 - Coveralls

Thanks @fredrikroos I will review this over the weekend. It does look like there are some merge conflicts now. Sorry for the delay in reviewing.

I rebased this on top of the current master, but can't share it in this PR, so here's a link to a commit in my fork: nickolay@9e9da81

This is roughly how I "rebased":

$ git reset --hard face972 # this PR's base
$ cargo fmt
$ # Some files were not reformatted in the PR, so undo the changes in them:
$ git checkout -- src/dialect/mod.rs
$ git checkout -- src/sqlast/value.rs
$ git commit -am .....
$ # Produce a patch without formatting changes:
$ git fetch upstream pull/28/head:join-support
$ git diff HEAD..join-support > join-support.patch
$ git checkout master
$ git apply join-support.patch
$ # Fixed conflict in the keywords list

@fredrikroos I fixed the merge conflict using the web editor. There is now one failing test - could you take a look please?

Thanks @fredrikroos I merged this manually and fixed the failing test