Workaround an issue with headers in httr.

Currently `httr` drops the `Content-type` header [in some
cases](https://github.com/hadley/httr/pull/147), causing many requests to
fail. This works around the problem by taking advantage of some case
sensitivity in header name handling.
Upgrade docs to roxygen 4.0.2
Use donttest in examples
Document all arguments
Use travis
Asserthat is now on cran
Add some github examples.
Closes #23
Simplify the signature of `query_exec` and `insert_query_job`.

The only information needed for a BQ query is (1) a query string and (2) a
project ID to bill the query to. This change simplifies `query_exec` to
only *require* these two fields, and packages the rest into
`destination_table` and `default_dataset`.

This is a decidedly breaking change for both `query_exec` and
`insert_query_job`; we probably want to bump a version number before merging
this. This also updates all references (I could find) to use the new
`query_exec` signature.

Also update all documentation.