Add scoped helpers environments from file-utils

Base generator now expose `.src` and `.dest` who're file-utils
environment scope as the sourceRoot and destinationRoot. This will allow
access to more file helpers method, and also more transparent process
for filtering, transpiling and validation (conflicter).
Lots of fixes to the help option.

Previously, running something like `yo angular:directive --help` would only show an error due to the missing required _name_ argument. This was not very helpful for the user.

I’ve changed this so that when providing the help option, the required arguments are added, but no error is raised if the argument is not provided. This results in a clean and consistent handling of the help option. Now when running `yo angular:directive --help`, the standard help/usage screen is shown, listing the options and arguments.

In addition to that, I’ve enhanced the help/usage function to list the arguments both in the usage line and also as an additional table similar to the options table. Let me know if that format is OK to keep like that, I tried to keep it similar to format used by the options.

As a byproduct, I had to add the _namespace_ to the _options_ object in `env.create`, since it’s used for displaying the correct name of the generator in the usage line. With the previous version, a call to `yo angular:directive --help` resulted in the usage line `yo directive`. Using the namespace in this place ensures that the correct name is displayed in the usage line.
Merge pull request #398 from nwinkler/help-improvements

Help improvements
Merge pull request #378 from SBoudrias/yo.file

Add scoped helpers environments from file-utils
Prevent the end event from firing multiple times.
Merge pull request #402 from timrwood/single-end

Adding failing tests for end event firing multiple times when copying a file more than once.
Use `setImmediate` if available

Node.js 0.12 deprecate `process.nextTick`, so we use setImmediate when
available, fallback to process.nextTick otherwise.
Join `sourceRoot` and `destinationRoot` logic together

Logic for these method was split in base.js and actions.js. Now they're
merged back together.
Resolve destinationRoot in prepCopy.

prepCopy resolves the `source` argument against the Generator's
configurable `sourceRoot`. This changelist adds the same behaviour
to `destination` and `destinationRoot`, respectively.
Merge pull request #401 from Schoonology/master

Resolve destinationRoot in prepCopy.
Make sure destinationRoot is a resolved path

Using relative path may not always be consistent with the context.
Fix global modules detection

Now that env.js became `env/index.js`, global mobules are relatively 5
levels up from it.
Fixed cosmetic relative shown by conflicter due to #378
Merge pull request #413 from jdespatis/cosmetic-relative-path-log2

Fixed cosmetic relative shown by conflicter due to #378
0.14.0
Moved changelog to GitHub releases: https://github.com/yeoman/generator/releases
Make sure npm test command don't exit with non 0
Merge pull request #416 from SBoudrias/travis

Make sure npm test command don't exit with non 0