Provide switch to turn on Vagrant's ssh forwarding

The switch is an environment variable, PDAL_VAGRANT_SSH_FOWARD_AGENT,
which, when set to any value, turns on ssh.forward_agent in the
Vagrantfile. More information about ssh configuration options can be
found in the [Vagrant
docs](http://docs.vagrantup.com/v2/vagrantfile/ssh_settings.html).

To turn on ssh forwading automatically when you boot your PDAL Vagrant
VM, put the following in your bash profile:

  export PDAL_VAGRANT_SSH_FORWARD_AGENT=true

It seems to me as if environment variables (with a PDAL_VAGRANT_ prefix)
might be the cleanest way to provide per-developer configuration for the
Vagrantfile. Other suggestions for these customizations are welcome.
Merge pull request #308 from gadomski/vagrant-ssh-forward-agent

Provide switch to turn on Vagrant's ssh forwarding
Fix seek for compressed BPF files.
More flexible point list parsing for "pdal info"
Merge pull request #312 from PDAL/pointlist

More flexible point list parsing for "pdal info"
Merge pull request #309 from PDAL/bpf

Fix seek for compressed BPF files.
Customize Vagrantfile with environment variables

This patch adds the ability for some `PDAL_VAGRANT_` prefixed
environment variables to customize the behavior of PDAL's Vagrantfile.
The general policy is to provide a 'switch' environment variable that
enables or disables a given behavior when the environment variable is
set to *any* value. For example, the variable
`PDAL_VAGRANT_DISABLE_NFS`, predictably, disables nfs mounting. A few
special cases actually use the value of the environment variable for
customization, usually some sort of integer value (e.g.
`PDAL_VAGRANT_VIRTUALBOX_MEMORY`).

This patch adds the following environment variables:

- `PDAL_VAGRANT_PORT_80_FORWARD`: set or disable forwarding from guest
  port 80. If an integer value, set the host port. If non-integer or
  zero, disable port forwarding for guest port 80.
- `PDAL_VAGRANT_VIRTUALBOX_MEMORY`: set or disable a custom memory value
  for a VirtualBox vm.
- `PDAL_VAGRANT_VIRTUALBOX_CPUS`: set or disable a custom cpu value for
  a VirtualBox vm.
- `PDAL_VAGRANT_VIRTUALBOX_IOAPIC`: set VirtualBox ioapic value.
- `PDAL_VAGRANT_VIRTUALBOX_ENABLE_GUI`: set to any value to turn on the
  VirtualBox gui.
- `PDAL_VAGRANT_PRIVATE_NETWORK_IP`: set or diable a private network IP
  for the vm.
- `PDAL_VAGRANT_DIABLE_NFS`: disable nfs mounting

The following values were added previously in
35b355f39b5d88801eeb80dfe038df05ed6c72fb, and are included here for
completeness:

- `PDAL_VAGRANT_SSH_FORWARD_AGENT`: enable ssh agent forwarding.

This patch closes #311.
Merge pull request #313 from PDAL/issues/311-configure-vagrant-with-envvars

Customize Vagrantfile with environment variables
point to my local PCL install
Merge branch 'master' of github.com:PDAL/PDAL
Add rxp reader test