Increase the timeout for the inttest/proto_gpb

On my slow old netbook with a 1.6GHz CPU, this inttest
takes about 40 seconds to run, of which network activity
is about 3s, the default timeout of retest is 30s.
Add a generous margin to avoid unnecessary timeouts.
Merge pull request #399 from tomas-abrahamsson/gpb-inttest-timeout

Increase the timeout for the inttest/proto_gpb
Update reference to installation notes
Merge pull request #403 from sdepold/patch-1

Update reference to installation notes
Consistently format export attributes
Cache code paths to reduce filename:absname/1 calls

rebar_utils:expand_code_paths is called enough times that the amount of
code path entries to be transformed to absolute paths leads to an
abundant number of filename:absname/1 calls. To fix that, cache
previously converted paths in a set.