Don't move point when clearing or highlighting test results

When running tests with cider-test failing tests are highlighted in the source
files, or if tests are passing again previous highlights are cleared. The code
that does this is using the `cider-find-var` function to find the buffer for a
vars under test and then does it's job.

`cider-find-var` used to move point to the line at which the var was found. This
is a problematic side effect, because code using `cider-find-var` to find the
buffer can't use save-excursion because it doesn't know which buffer to safe
beforehand.

This PR removes the `goto-line` in `cider-find-var` and returns just the buffer
without any side effect. The `cider-company-location` uses similar code as
`cider-find-var` but with `save-excursion` wrapped around it.
Merge pull request #717 from r0man/stop-moving-point

Don't move point when clearing or highlighting test results
Simplify a bit of completion code
Add a cider-stacktrace-print-level defcustom

Controls the depth of printing of a stacktrace cause's data.  Can be used to
prevent circular data structures from causing exceptions not to display.
Merge pull request #738 from hugoduncan/feature/add-print-level-to-stacktrace

Add a cider-print-level defcustom