Show original statement if assert or refute fails

If the asserted statement evaluates to false, show the unevaluated
statement in the assertion error output message for more context about
the failure. Refute also now follows similar semantics.

Fixes #1276
Slight clean up on erlang tests
Merge pull request #1299 from vanstee/assert-statement-if-boolean

Show original statement if assert or refute fails
Fix typo on assertion message
Fixed typos in file.ex docs
Fixed ex_doc path in README
Merge pull request #1302 from jwarwick/typos

Fixed doc typos
Optimize temp_vars lookup
Include description and improve formatting

Switching from `AssertionError` to `ExpectationError` gives us a nicer
error message and allows us to easily include the incorrect result as a
description.

Fixes #1276

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>
Clean up ExUnit tests