Improve support for tramp source buffers

Ensures load-file doesn't use the tramp location of the file for the
filename passed to the nrepl load-file op (which gets compiled into
debug information).  This caused some issues when using the same repl
with source files both locally and over tramp.

Uses an explicit buffer to determine the tramp prefix rather than the
find-tag-marker-ring.  Changes the signature of cider-jump-to-def-for to
add a buffer argument.

Changes cider-jump-to-def-for to look for a tramp path before a local
path.
Remove leading whitespace in doc strings
Merge pull request #486 from hugoduncan/feature/tramp

Improve support for tramp source buffers
Make cider-doc-handler work synchronously, and rename it
Extract `cider-doc-buffer-for'

And use it in `cider-complete-at-point'.
Make cider--jump-to-def-eval-fn work synchronously
Move find-tag-marker-ring insertion to cider-jump-to-def-for
cider--jump-to-def-eval-fn-1: signal errors
Define cider-company-location

It's a hack, but a simple one.
Define cider-company-docsig
Mention company-mode and company-cider
Upcase the argument name
Add Changelog entry
cider-company-location: check explicitly for namespace error message
Merge pull request #490 from dgutov/company-capf

Support company-mode
cider-doc-buffer-for: return nil when response was empty
cider-ido-read-var: add a space to the prompt
Merge pull request #491 from dgutov/doc-tweaks

Doc tweaks
Fix function indentation
Extract cider-var-info and cider-get-var-attr