- try to not rewrite on update if there's no change
- handle 'mode' the same way as in the jinja2 recipe

The expected usage of zc.buildout.download.Download() is that the caller is
supposed to remove the temporary files (cf [1] [2])

[1]: https://github.com/buildout/buildout/blob/3ed7c06bf7be594dcb3c906c2517ee438f059251/src/zc/buildout/download.txt#L55
[2]: https://github.com/buildout/buildout/blob/3da22a7dacb9a4d1f771e7763e014214c31d935b/src/zc/buildout/download.py#L171-L176

See merge request !5

setuptools 42.0.0 prints a warning:

   WARNING: The easy_install command is deprecated and will be removed in a future version

use a test normalizer to ignore this warning in doctest.

Local variable `template` is used for different things, it is bad. Use different variable name for different thing.

compile function takes a lot of time, thus this change saves time when the same source is used many times in apache frontend SR.

__init__ should only check options. By doing the actual work in install/update,
it is now possible to use a template that is produced by another part.

'mode' was implemented like 'umask' but it has never been used this way,
and the 'umask' option is deprecated:
- remove support for unused 'umask' option
- change 'mode' to actually apply the wanted mode, regardless the current umask
- the default mode changes: if 'mode' is not given, the resulting file is only
  executable (actually 0777 & ~umask) if the content starts with '#!',
  else its mode is: 0666 & ~umask

This is an incompatible change. For example, you would now write:

  import easy_install zc.buildout.easy_install

instead of:

  import zc zc.buildout.easy_install