Use a generic exception in _check_call.
Merge pull request #292 from memmett/memmett/check_call_raise

Use a generic exception in _check_call.
Add update mode

This mode allows a user to append to nodes in the YAML file.

This can be useful when programmatically building up a set of configure
commands, for example, instead of resorting to a Python hook file.

Example use:

```
  - when: pyver == '3.4'
    name: configure
    mode: update
    extra: ['--without-ensurepip']

  - when: python_framework
    name: configure
    mode: update
    extra: ['--enable-framework=${ARTIFACT}']
```
Add a test for the update mode
Working test for update
Updated docs for update mode
Merge pull request #294 from hashdist/ahmadia/add_update_mode

Add update mode
Add parameter set/override from command line

Parameters can now be set or overridden from the command line using the syntax:

x=y

An unlimited number of parameters can be set.  A current limitation is
that a parameter set at the command line may not contain the '=' sign,
as this is used to parse the split between the two parameters.

Some examples:

hit build -v profile.yaml HOST_MATLAB=~/bin/matlab
hit build -v scipy-stack.yaml pyver=3.4 PATH=/usr/local/bin:/usr/bin:/bin
hit currently relies on a Python2

This will hopefully fail more meaningfully if a user doesn't have Python 2 as their default Python.
Revert "hit currently relies on a Python2"

This reverts commit 9aad6b4c381ab888afc94949d2a3154a3d19a03c.

This breaks on OS X
Merge pull request #297 from hashdist/ahmadia/add_parameter_keywords

Add parameter set/override from command line
Don't assume presence of parameters key in profile