Describe using PyPI/pip to distribute & install extensions

From discussion on gh-5742
fix delete_checkpoint errback name
try wheels for faster dependency installation
Merge pull request #5744 from takluyver/doc-extensions-pypi

Describe using PyPI/pip to distribute & install extensions
Add simple tests for kernel spec machinery
Merge pull request #5598 from takluyver/kernelspec

Kernel registry (IPEP 25)
Merge commit '55a3270aa4724bbea6a0ae0088f770c46aabf04a' into patch_2565_sidewindow
Introduce a doclkable widget that will substitute "all magics" menu.

Features:
a. filter by regexp
b. filter by class
c. double click pastes
d. push buttons to paste and run magics
Remove "all magics" menu stuff.
Move Maigic Helper into separate class/module/file
Eliminate main window logic from MagicHelper.

The only remaining logic in MagicHelper was calling kernel upon update.
Moved the call to main window using new special signal 'readyForUpdate'
Upon receival of this signal main window will query the kernel and call
MagicHelper.populate_magic_helper()
Merge pull request #5745 from minrk/travis-cache

try wheels for faster dependency installation on travis
add utils.path.ensure_dir_exists

Ensures that a directory exists,
and protects against a common race condition when multiple
processes try to create the same directory.
better error message when path exists but is not dir
change default dir permissions to 755

777 is a weird default
test ensure_dir_exists
Merge pull request #5739 from minrk/ipython-dir-race

add utils.path.ensure_dir_exists
Added more documentation for MagicHelper.
removing gitwash_dumper

we don't use this file, and it had an offending race condition for
makedirs
Merge pull request #5750 from ivanov/raec-cnodiiton

Remove obsolete gitwash_dumper tool