shorter build dirs

move `_logs` and `_cover` to `_build` dir

to `as` used to generate them

Do not automatically check plt

Fix hg diffing

- fetch more complete refs for tags (may download more history than
  required, but it works)
- Fix comparison of tag distance by using lists everywhere

use rebar_utils:sh for git and hg commands to have better errors and log

use rebar_file_utils:mv instead of copy for fetched source

`cover` task

`as` delegates task handling to `do` provider

pre-purging before checking to facilitate long do chains

source files in the `compile` key of `module_info/1` so `rebar cover`
struggles locating source in some instances. in these cases a warning
is printed and crappy coverage info is written rather than none at all

`rebar.config` or via the `-c\--cover` flag given to the appropriate
task) from the `eunit` and `ct` tasks and add a `cover` task to
write coverage analysis to disk

make base_dir for a run include the profiles in path, link to shared dep

Some free tests from a debugging session

The session was unfruitful, but I guess the tests were not necessarily

don't replace non-string list with empty list in profile merge

Documentation is maintained on www.rebar3.org

reset state between tasks run with `do`

has the same outcome as `rebar3 foo && rebar3 bar && rebar3 baz`

fixes #191

Port env var expension fix from rebar2

add edoc provider

Support mercurial (hg) dependencies