preload_module function for IDEs, to control which modules to load on startup, refs #102
authors change
use coverage instead of pytest-cov
remove pytest-cov from tox.ini requirements
forgot the coverage dependency when removing the pytest-coverage
don't erase coverage data
Merge pull request #215 from davidhalter/cov

Use coverage differently
update debian installation notice, cc @p1otr
Removed import hack. Relative imports changed to absolute. Closed #160.

Description:
- Absolute import commands "from jedi import XXX [as yyy]" stay unchanged. (except the note)
- All relative import commands "import XXX" replace by absolute imports by 3 ways:
  * "from jedi import XXX"  # preferred if possible
     # unchanged code
  * "import jedi"
     # ensure that XXX is imported just in time later by other module, usually by jedi.api
     replace "XXX." by "jedi.XXX.", It completely removes the cycle.
  * "import jedi.XXX"
     replace "XXX." by "jedi.XXX."
- These changes combined in order to minimize adding "jedi." and to prevent cyclic import.
- The aliases "er" and "pr" for evaluate_representation as parsing_representation are
  so frequent, that they should be preserved, but in the "recursion" module was necessary
  to replace it once because decorators defined in "recursion" must be imported very soon
  without cyclic dependencies.
- Modules docstrings, dynamic, evaluate and recursion are not imported also by all
  modules that access them by "jedi.xxx". This is also due to preserve "er" alias.
- Everything relevant is imported by "import jedi". Therefore the order is indepentent
  on the usage of jedi, whether a "jedi.X" or "jedi.Y" is imported first or later.