explicit warning about the dangers of compress_ptr
pep8 fixes

blosc/toplevel.py:37:14: E261 at least two spaces before inline comment
blosc/toplevel.py:41:56: E702 multiple statements on one line (semicolon)
blosc/toplevel.py:44:13: E261 at least two spaces before inline comment
blosc/toplevel.py:80:77: E502 the backslash is redundant between brackets
blosc/toplevel.py:151:77: E502 the backslash is redundant between brackets
blosc/toplevel.py:159:1: E302 expected 2 blank lines, found 1
blosc/toplevel.py:222:17: E123 closing bracket does not match indentation of
opening bracket's line
blosc/toplevel.py:225:68: E502 the backslash is redundant between brackets
blosc/toplevel.py:233:1: E302 expected 2 blank lines, found 1
blosc/toplevel.py:311:72: E502 the backslash is redundant between brackets
blosc/toplevel.py:364:1: E303 too many blank lines (3)
blosc/toplevel.py:368:62: E502 the backslash is redundant between brackets
adding a .mailmap file

This give improved legibility when using git shortlog by combining multiple
usernames/emails into a single line:

```shell
$ git checkout master
Switched to branch 'master'
$ git shortlog -s -e -n
    31  Francesc Alted <faltet@pytables.org>
    20  Valentin Haenel <valentin.haenel@gmx.de>
    14  Francesc Alted <francesc@continuum.io>
     7  FrancescAlted <francesc@continuum.io>
     2  FrancescAlted <faltet@pytables.org>
     1  cgohlke <cgohlke@uci.edu>
     1  gcmalloc <gcmalloc@gmail.com>
$ git co mailmap
Switched to branch 'mailmap'
$ git shortlog -s -e -n
    54  Francesc Alted <francesc@continuum.io>
    21  Valentin Haenel <valentin.haenel@gmx.de>
     1  cgohlke <cgohlke@uci.edu>
     1  gcmalloc <gcmalloc@gmail.com>
```

The discrepancy in my number of commits is caused by the commit that introduces
the mailmap.
Merge pull request #21 from esc/mailmap

Adding a simple mailmap file
Merge pull request #19 from esc/docstring_compress_ptr

explicit warning about the dangers of compress_ptr
Merge pull request #20 from esc/pep8/toplevel.py

Minor pep8 fixes.