Backport AxographRawIO into 0.7.x maintenance branch (#682)

* AxographRawIO WIP: Copy of example with working tests

* AxographRawIO WIP: RawIO and IO working

Dependency on axographio package eliminated! Still missing multi-segment support, and there are new metadata available from AxoGraph files (events, epochs) that should be made accessible to Neo. Also need lots and lots of tests.

* AxographRawIO WIP: Python 2 support

* AxographRawIO WIP: Implement Events and Epochs

* AxographRawIO WIP: Implement multi-Segment for episodic files

* AxographRawIO WIP: Fix for old formats

* AxographRawIO WIP: Add some basic annotations

* AxographRawIO WIP: Filter events/epochs by time when lazy loading

* AxographRawIO WIP: Comments

* AxographRawIO WIP: Another condition for episodic

* AxographRawIO WIP: Add rec_datetime and file_datetime

* AxographRawIO WIP: Add tests

* AxographRawIO WIP: Return None for unsupported objects

* AxographRawIO WIP: Re-alphabetize

* AxographRawIO WIP: Remove old non-raw code

* Fix divide by zero in benchmark_speed_read_signals

If read times are fast enough (especially in Python 2, perhaps because of a lower precision performance counter?), division by zero can happen when calculating speed in `benchmark_speed_read_signals `. This commit catches this problem.

* AxographRawIO WIP: Documentation

* AxographRawIO WIP: PEP8

* AxographRawIO WIP: More PEP8

* AxographRawIO WIP: Remove spaces from test file names

* AxographRawIO: Fix minor typos and poor variable name choice

* AxoGraphRawIO: Catch bad datetime string parsed from notes

* AxographRawIO: Fix typo in comment

* AxographRawIO: Catch rec_datetime not set if datetime_string is empty