Release 1.3.1 - Palo Alto support, and more better SSH!

+ General changes

  - New contrib package for optional extensions to core Trigger
    features, `~trigger.contrib.commando.CommandoApplication` being
    the first.
  - Remove legacy mtsync check from bin/fe.
  - Conditionally import MySQLdb so we can still do testing without
    it.

+ The following changes have been madw within `~trigger.acl.parser`,
  which provides Trigger's support for parsing network access control
  lists (ACLs) and firewall policies:

  - :bug:`72` Bugfix in `~trigger.acl.parser.TIP` where an invalid
    network preifx (e.g. '1.2.3.1/31' would throw an
    ``AttributeError`` when checking the ``negated`` attribute and
    shadowing the original ``ValueError``.

+ The following changes have been made within `~trigger.cmds`, which
  provides an extensible, developer-friendly interface to writing
  command exeuction adapters:

  - Added ``with_errors`` argument to `~trigger.cmds.Commando`
    constructor to toggle whether errors are raised as exceptions or
    returned as strings.
  - Allow timeout to be set as a class variable in
    `~trigger.cmds.Commando` subclasses, preferrring timeout passed to
    constructor in `~trigger.cmds.Commando` subclasses.

+  The following changes have been made within `~trigger.netdevices`:

  - Refactor how we id Brocade switches for startup/commit (fix #75)

    * It's assumed that all Brocade devices all act the same;
    * Except in the case of the VDX, which is treated specially.

  - Simplified how ``startup_commands`` are calculated
  - Disable SQLite loader if sqlite3 isn't available for some reason.
  - Prompt patterns are now bound to `~trigger.netdevices.Vendor`
    objects object when `~trigger.netdevices.NetDevices` is populated.
  - `~trigger.netdevices.Vendor` objects now have a ``prompt_pattern``
    attribute.
  - All prompt patterns are now defined in ``settings.py``:

    * Vendor-specific: :setting:`PROMPT_PATTERNS`
    * IOS-like: :setting:`IOSLIKE_PROMPT_PAT`
    * Fallback: :setting:`DEFAULT_PROMPT_PAT`

+ The following changes have been made within `~trigger.twister`,
  which provides Trigger's remote execution functionality:

  - Added CLI support for Palo Alto Networks firewalls!
  - SSH Async now enabled by default for Arista, Brocade.
  - :feature:`54` Moved static definition of commands permitted to be
    executed when specified in a users' ``~/.gorc`` file into a new
    configuration setting :setting:`GORC_ALLOWED_COMMANDS`. The file
    location may now also be customized using :setting:`GORC_FILE`.
  - :bug:`68` Fix host lookup bug in `~trigger.twister.TriggerTelnet`
    causing telnet channels to crash.
  - :bug:`74` Fix error-detection for NetScaler devices.
  - Enhanced logging within `~trigger.twister` to include the device
    name where applicable and useful (such as in SSH channel
    debugging).
  - All ``execute_`` functions have been simplified to eliminate
    hard-coding of vendor checking wherever possible.
  - Beginnings of reworking of Generic vs. AsyncPTY SSH channels:

    * Most vendors support async/pty with little problems.
    * This will become the new default.
    * New execute helper: `~trigger.twister.execute_async_pty_ssh`
    * New error helper: `~trigger.twister.has_juniper_error`
    * Arista now uses `~trigger.twister.execute_async_pty_ssh`
    * A ``NetScalerCommandFailure`` will now just be a
      `~trigger.exceptions.CommandFailure`

+ Documentation

  - Updated README to callout CSV support.
  - Updated README to reflect branching model.
  - Updated supported vendors, and no longer promising NETCONF
    support.